I noticed that too with sequential sounds. I don't know the coding reasons behind it, (maybe it's the infamous
memory leak) but I know the effects and this is what I was able to diagnose and come up with:
1. When you exit Orbiter and start a scenario again, the sound will be delayed as if the internal "counter" kept
running while you were in the launchpad -> Solution: Exit Orbiter completely, every time you wish restart a scenario
that uses a sequential file.
2. If you use time accaleration while using a sequential sound the same clip will be repeated after you go back to
normal time. Every time. I have no solution for this one.
Other than that it should all work fine. Both of these issues should be gone in the new OrbiterSound version, coming
out when the new Orbiter version also comes out

Hope it helps, cheers.