Asterisk and Slackware 12.1
I upgraded my Linode to Slackware 12.1, at the same time doing a kernel upgrade, and upgrading Asterisk to version 1.4.22. When I did this, I discovered that the prompts sounded like crap. The actually call quality was fine once it was connected to my ATA at home. It also did not effect tt-rickroll, a gsm I encoded at home using a gstreamer/sox script, music on hold, or playback of actual voice mail files (the prompts were still messed up). This puzzled me for days...
Eventually, after rebuilding the Asterisk package, switching from ztdummy to dahdi_dummy, manually extracting the sound files, and giving my phone number to some #linode guys, I stumbled on this post to asterisk-users, which referenced this post. These posts said that the problem lied in the fact that I was using gcc-4.2, which had some optimization bugs. Turns out that Slackware bumped gcc from 4.1 in -12.0 to 4.2 in -12.1. With an enormous amount of skepticism, I rebuilt the package using "DONT_OPTIMIZE", and wouldn't you know, it worked!