Turns out there’s a bug in the optimizations on the GCC compiler. 

Previously, was running 4.1; an ubuntu upgrade took it to 4.2 however, the audio on any gsm messages (voicemail, intros, etc) all sounded garbled.  Anything wave played fine.

A bit of googling found this is a problem affecting many users. 

Quick fix…

cd /usr/bin
ls -lsah gcc

If you see … gcc -> gcc-4.2


rm gcc
ln -s gcc-4.1 gcc
ls -lsah gcc

should now show … gcc -> gcc-4.1

then off to your asterisk src folder and recompile

make clean
make menuselect
make install