Here is the sound solution for the RtCW sound problem (thanx to FonFon):
Replace the line "set normal "cl_freezedemo 0; timescale 1;echo ^2Normal; vstr normal2;"" with "set normal "cl_freezedemo 0; timescale 1; s_stop; echo ^2Normal; vstr normal2;"" in the Return to Castle Wolfenstein configs of Seismovision. This will also be in the next release of Seismovision.
The command "s_stop" resolves the sound problem.