Why is sound on my Android suddenly increasing? I am running Android 2.3 on a Samsung Galaxy S 2.

Recently, when I play music, at some random time, the volume will suddenly increase. It's hard to put an exact measurement on the increase, but I'd estimate it's about 20 to 30% louder.

It's quite jarring, and if my volume is already up near maximum, it can be a little painful.

This happens with any application that plays MP3s, so I don't believe it's application specific.

It also happens without any action on my part. When the volume jumps up, I'm not holding my phone or using any interface, or touching any buttons. It's not that volume buttons are accidentally being hit, either. It happened just now, and my Android is lying on the table beside my laptop, without anything touching it.

Is this a bug of some kind?
It could most likely be a bug. I suggest you upgrade your Android via Kies or manually.

Galaxy Nexus is known to have a similar software issue with volume: http://techie-buzz.com/mobile-news/galaxy-nexus-to-get-a-software-update-to-fix-the-volume-bug.html Perhaps it could be the same with your S2, especially since it is on older Android version.