SoundEffectInstance resumes incorrectly the second and subsequent times. The resume position is computed by the resume method as: resumePosition += pausedPosition. This is correct only if resumePosition is 0 (i.e., the first time resume is invoked). The right formula is resumePosition = pausedPosition.
FYI. In case anyone comes looking. Also, Adobe doesn't seem to be aware of this bug. Couldn't find any reports in their public bug tracker.