is it okay to use a bind that turns game volume on or off? sometimes listening to glados 10 times a day can get annoying

If you wish to not hear the ingame Audio, its best to just plug in a set of headphones, and not use them, so the run can still have audio for verification reasons.

Cosmetic mods are allowed, right? Why not change the repetitive dialogue to something more entertaining?

Some people chose to delete most of the voice lines. This has no effect on the gameplay seeing as the game is based on triggers and not dialouge. It does not matter for verification reasons either, so yes, you can change them to whatever you'd like within reason.