Losing sound in instances

[Alram]

The last 2 times that I have been in an instance--1st Ragefire and 2nd Shadowfang, I lost all sound. Intermittently music would briefly play, but no other sound. The 1st time a reboot after the party wiped fixed the sound. The second time I exited the game while the rest of my party played on, but after a reboot there was still no sound. I hit esc, and went to "sound" and found"enable all sounds" to be unchecked! Once I placed a check in the box, sound was restored. What is going on here? Did this ever happen to anyone else? I have never had this problem at any other time or with any other apps on my computer. Both times it was with the same toon.