As a request from some friends I created a sound pack that lowers the volume of portal sounds. All portal sounds were lowered by -20DB (travel.trigger.portal,endportal).
This pack can be used even on older minecraft versions winouth a problem keeping in mind that soundfile locations weren't altered in a few years.
I do allow the modification/distribution on other websites or platforms only if I get credited and have the link to the project visible for download.