SIPp 1.1rc4 brings RTP play capabilities. SIPp is a free Open Source test tool / traffic generator for the SIP protocol. The project is very active and is largely used in the SIP community.
The new pcap play feature (contributed by Guillaume Teissier) allows to send RTP streams and RFC2833 DTMFs along with the SIP traffic. This greatly widen the scope of SIPp and allows SIP compatible equipment provider to test their implementation better than ever. For free.
An example of a SIP scenario with RTP play is available here: http://sipp.sourceforge.net/doc1.1/reference.html#UAC+with+media
SIPp's project homepage is: http://sipp.sourceforge.net/
SIPp sources (for HPUX/Linux) or Windows executable can be downloaded from: http://sourceforge.net/project/showfiles.php?group_id=104305&package_id=119322