I ran into a weird issue, where I wasn't getting any sound in some apps like Hooked on Phonics and Sky Fish. I tried reinstalling those apps, quitting all other apps, etc. as some suggested, but they didn't work for me.
Then it occurred to me, that some software may not have been written correctly to work with the Lock Rotation / Mute switch; what if the developer assumes the switch is only used for Mute? I decided to test out that hypothesis, so proceeded to my Settings screen, changed my current selection back to Mute (which is the iOS default), and Voila, it worked!!!
What's more interesting, is that once the "problem was fixed", I was able to go back to the Lock Rotation setting, and vice versa, and those apps were no longer having issues. So, I am thinking that there could be some bug in the iOS - possibly with its Restore feature, as this iPad was recently restored from its iCloud backup - that is not properly setting some of the internal bits regarding the sound device availability (whatever that's linked to the Lock Rotation / Mute physical switch); it's possible that that switch was on the Mute position when it was restored (but then why didn't other apps have issues?).
Anyway, I hope this is helpful to those having a similar issue.