Agree with the previous comment. This is only going to become an increasingly annoying issue. Need to raise awareness of this to get it fixed.
I've been running into this problem too. I sincerely hope this will be fixed with the introduction of official HD texture pack support, it is becoming a real issue for my server as well. It's hardly invalid considering it's becoming official.
Screenshot of me sitting in the nether. FPS is good until I drop down near the lava, then it just crashes and sound jumps up massively as seen here.
I personally have to disagree that 50MB is sufficient even for a vanilla pack, especially now. Once you start considering that resource packs can add new sounds, change sounds, and change the music in particular, you can get over 50MB very easily.
One of my vanilla resource packs is sitting at 65MB and this is mainly due to the addition of new sounds and music. If music in resource packs is a supported feature, then it should be fully supported.
I'd also like to add that not even the default Minecraft assets are under 50MB (it's sitting at 87MB on my harddrive, unzipped at least).