If you are experiencing crackling and choppy music on hold and you are using Asterisk 1.4.x below is the fix that Ace Communications has provided:

1. Edit /usr/sbin/safe_asterisk
2. Find the line for the ASTARGS – should look something like this: ASTARGS="$ -vvvvvvvvvvg
3. Add an 'I' ('I' for India) to it for the timing ASTARGS="$ -vvvvvvvvvvgI
4. Save the file and exit
   
5. Edit /etc/asterisk/asterisk.conf
6. Uncomment [options]
   
7. Add the following to the [options] section of the file: internal_timing=yes
8. Save the file and exit
9. Stop/start Asterisk

See: http://forums.whirlpool.net.au/forum-replies.cfm?t=1035202
Courtesy: Ace Communications