Music Buttons Gone Haywire

I don't know if this would fix your problem, but I would name your buttons something like "button_1" through "button_9", and instead of calling just "button", call "button_*". The "*" is a wildcard, meaning that it can take the place of any string coming after it. You can read more about that here

I remember seeing this happen with a deathrun map. Here is my suggestion:

OnDamaged s1 PlaySound 3 <==== Adding a delay before it plays. Since you have it set to play a sound and stop a sound at the same time, adding a delay would prevent it from playing two sounds at the same time (even if it is a single frame).

After re-reading your post. If you hit a button before the 5 minutes are up, does the problem occur, or is it after the 5 minutes? Either way, I would make it so that when you hit a button, all the buttons disappear (disable) for 300 seconds instead of locking them.

Did you try "ent_messages_draw 1" and "developer 2" to see what's going on? Both show what's being triggered in the map, visually and through console. (I use both regularly, but I can't remember what each does specifically!)