Edit: In 8.2 you can no longer have custom audio in WOW. Seriously, #*$% off Blizz this was an issue in beta yet you let it through. Add this to the hamhanded way they are forcing graphics options on players, I'm done.
Edit: June 16, 2019, added Drustvar replacement music - check out the latest BfA file under "Files"

Over the many years /timeplayed, whenever I have found a particular song or sound I eventually didn't like, I eventually changed it. I'm also a music buff so i was able to dig pretty deep into many genres (such as New Age, classical, film, techno, trance, etc) and selected replacements for timelessness. Must be said here, the vast majority of music genres, I like!
I also collected pretty much all sound customizations that eliminated or dampened certain annoying sounds. Such as worgen sniffing. Making MoP's faction hub quiet. A pup whimpering. Removal of that dude's spammed speech in old Dal. Things like that.
All told, 1.6 gigs. [Edit - this is larger now, I occasionally add new files]

This isn't an addon. Here's how to "install" (not the usual yadda yadda)
There are 5 zip files under the Files tab (above) named "Classic thru Legion 1 of 4" (through 4 of 4) and also "BfA" that are a collection of mp3s and .ogg files with the required folder structure. As they are over 1.6 gigs together - well over the 1.2 limit by Curse - I had to split them. Sorry.
Once you download all 5 parts you can browse through the music before you extract into WOW to get an idea what the choices are.
All five files should be extracted into the World of Warcraft\_retail_\Sound folder. This is the only tricky step. Note this file structure changed in BfA (added "_retail_")

MAKE SURE you end up with this folder/path structure - here are some examples:
C:\Program Files (x86)\World of Warcraft\_retail_\Sound\Ambience\WMOAmbience\StormwindDay.ogg
C:\Program Files (x86)\World of Warcraft\_retail_\Sound\Doodad\BE_MagicalKnickKnack04Loop.ogg
C:\Program Files (x86)\World of Warcraft\_retail_\Sound\Music\Legion\MUS_70_DalaranHalls1_B.mp3

Then when you restart WOW the custom sound files will be used instead of default. You can always delete or change any files, put in a song you want, changing the song title to the corresponding WOW title, or just delete everything in Sound and start from default. Note: You can also just change the folder names ever so slightly to toggle them off (restart needed)

Basically, in case you don't know this, it's sorta counterintuitive, but if the Sound folder is empty, it uses all the default wow sounds, and if anything is in it, it will be custom loaded. All of WOW's music is stored in files you can't normally access.

UI
Added a brief Mass Effect Galaxy Map sound when opening a trade skill. Blood elf female moans suggestively when crit.

Changes by Expac
I'm only giving a brief overview, as there are way too many tracks to document individually. You will notice sometimes there are extra songs laying around with a suffix on the end, I often swap songs in and out. For example, in the Stormwind city music folder I have an extra, fantastic song from Warcraft 3 Frozen Throne. It is titled stormwind02-momentHuman4x. If I edit out the Human4x at the end, it becomes the song WOW uses.

Classic
-Most music was left untouched
-Completely redid Stormwind ambient day and night, and also the music selection for Org/Stormwind/Darnassus and tavern music for most cities
-PVP/BG music changed

-Darkmoon Faire music changed
-Changed Flying Machine and Sky Golem sound to Half Life gunship sound

-Silenced or dampened many annoying pet sounds
-Changed 2H weapon sounds to be more impactful
-Gun sounds changed

2.0 BC - 3.0 WotLK
Few if any changes. it's flawless. Really. Oh yes - Silvermoon zone music changed.

4.0 Cata
Most music left untouched, except for the new goblin and gnome areas, which were mostly replaced with techno/industrial. Some ogre music changed to some dark industrial.

5.0 MoP
~30 music tracks altered
-Silenced Alliance/horde hub Shrine sounds in MoP
-Siege of Orgrimmar heavily altered

6.0 WoD
~30 music tracks altered, especially a lot of the out of place (IMHO) Edo's whimsical music. Remixed a couple stock tracks, BRF has some nice touches.

7.0 Legion
-Changed most bombastic, wearying Legion Dalaran music to something far subtler with a touch of Mass Effect Citadel and Bladerunner
-Valsharah and the druid music substantially changed, much of it to dark trance
-A LOT of Suramar music changed
-1 or 2 Stormheim tracks changed to WotLK's Storm Peaks

After Classic - Legion 1-4 I am adding new expacs as separate files as they come out. They do not replace the other older expacs or folders.

8.0 Battle for Azeroth
Upload into:
"C:\Program Files (x86)\World of Warcraft\Sound\Music\battleforazeroth"
-For Kultiras I left it mostly alone although I added a lot of gothy celtic music to Drustvar
-Extensive changes to Zandalar, from Peter Gabriel Passion and Star Wars Tatooine theme in Vol'dun to a lot of jungle trance in Nazmir and tribal music in Zuldazar.

Identifying what tracks I used
Look in the tags for each file. You can enable viewing them in Windows Explorer. If that fails (a few are blank) and Shazam returns nothing, just ask! I obviously do not claim to own any audio files.

DIY
If you would like to do something similar, download the CASC editor at
https://github.com/WoW-Tools/CASCExplorer/releases
and (required!) an updated listfile at https://github.com/wowdev/wow-listfile
TL;DR you extract the files from your WOW folder into another folder so you can go through it. That way you know the structure and what sound file is what. Wowhead also helps in finding a certain track, also youtube.

Glue Screen - Experts Only
Glue screen is wow's word for the music that plays when you start the wow client. Gets old fast!
BfA's launch music is sound\music\battleforazeroth\mus_80_beforethestorm_maintitle.mp3.

I included a folder "Glue Screen Music - SPECIAL." This folder is not part of the standard folder structure. This special folder has 7 folders numbered 1-7. These each have a glue screen track that I rotated. An easy way to customize this with no setup is to simply create a folder under Music called "battleforazeroth" and put one of the songs in the folders (or a song you choose) into the battleforazeroth folder.

That's an easy method. But you can also rotate different songs into it on a cycle you set.
To enable this, it gets complicated quickly. But basically, TL;DR i made a bat file (i.e. "wowsound1.bat" in notepad) and set it to run periodically through Windows Task Scheduler that changed the track every 30 minutes.
The bat file is one line:
robocopy "C:\Program Files (x86)\World of Warcraft\Sound\Music\Glue Screen Music - SPECIAL\1" "C:\Program Files (x86)\World of Warcraft\Sound\Music\battleforazeroth" mus_80_beforethestorm_maintitle.mp3
This injects a particular track into what the WOW client will play as the main music on the glue screen. Then I made another bat "wowsound2" and used "2" instead of "1"
robocopy "C:\Program Files (x86)\World of Warcraft\Sound\Glue Screen Music - SPECIAL\2" "C:\Program Files (x86)\World of Warcraft\Sound\Music\battleforazeroth" mus_80_beforethestorm_maintitle.mp3
and had Task Scheduler play 15 minutes after #1. So it rotated randomly. You can change the numbers around and alternate as you see fit. You might have to add an exclusion to your antivirus as normally they hate BAT files.
Double check the paths in the example by the way... It may be uploaded as being in the Music folder not Sound.

Future Changes
By the mid of each expac I will release a new zip of any music changes. As this isn't a normal addon, the Twitch client won't pick up when there's an update. My best recommendation is to "Follow" this Project.

Also check out my related project, Warcraft Custom Sounds, a huge collection of sounds for addons and Weakauras, at
https://wow.curseforge.com/projects/new-warcraft-custom-sounds

Warcraft Custom Music