Mixing two rtp streams of a voip call

I am trying to mix two rtp streams that I extracted from the pcap of a voip call (using rtpbreak).

Problem is when I mix them ( sox -m) . The audio from the two streams are never in sync. But when I play the streams using wireshark, it is in sync.

Procedure used

1. let the timestamp of the first packets of the 2 rtp streams(say A and B) be t1 and t2 (t1 > t2).
2. pad (t1 - t2) amt of time at beginning of A to make stream C
3. mix B and C.

It would be very helpful if someone could tell me how it is done in Wireshark. Thanks in advance