RTP stack should not alter the payload so the change of 0x1 to 0x31 sounds strange. What happens if you send a RTP packet in this way:
Code works fine on N95 and Event field has a value 0x1. Btw, are you waiting RRtpSendSource to call a callback function which tells that sending is completed before sending the next packet? Sending a new packet before the old one is completed might cause some strange behavior..
iSendPacket.SetMarker( 1 );
iPayloadDesC.Set(const_cast<TUint8*>(iSendPacket.WritePayload().Ptr()), iPacketSize, iPacketSize); //packet size is 160
payload.Append(TPtrC8((const TUint8 *)("\x1\x8A\x07\xD0"), 4)); //4
TInt tmp = iPayloadDesC.Length();