This resource pack adds an old modem sound as ambient.legacy_modem.dialup1 and isolates the basalt_deltas_click5.ogg sound as ambient.basalt_deltas.click5. Use the /playsound (id of the sound) to play those sounds.

Note: It does not passively add sounds to the ambient background. The sounds have been added only for people who use command blocks for their projects. So this data pack does not actually change the game.