Googled "sniff VOIP" and found a ton of relevant articles.

Funny thing was... the FIRST result was of our AO resident, Irongeek... and his tutorial
for sniffing VOIP using CAIN. Not sure if you saw his tutorial, if not, have a look at it:
Sniffing VoIP Using Cain


Also, what platform/OS you are using to sniff from?
Here are a couple apps:

TelRex
Realtime recording & Monitoring. TelRex CallRex - is a software-based VoIP call recording solution that supports integration with 3Com NBX, Cisco Call Manager, Mitel ICP, Nortel BCM, Artisoft Televantage, Avaya IP Office, Siemens HighPath, NEC NEAX, Shortel Shorewave, and Zultsys MX250.

Vomit
voice over misconfigured[1] internet telephones

VoIPong
VoIPong is a utility which detects all Voice Over IP calls on a pipeline, and for those which are G711 encoded, dumps actual conversation to seperate wave files. It supports SIP, H323, Cisco's Skinny Client Protocol, RTP and RTCP.

And a bunch more.....