On both my Mac systems (Macbook and iMac) the latest available version, 5.02, is using enormous amounts of CPU resources and is making other apps completely unresponsive, so it doesn't appear to just be eating up idle time. By the way it starts at a high level and just grows until the whole system is unresponsive is acting like a memory leak somewhere.
I've tried this on both Mac OSX 10.6.2 and 10.6.3. Express Scribe 5.02. Doesn't appear to matter what kind of sound file format (AIFF, WAV, FLAC, MP3). CPU resources are up around 98%. Closing Express Scribe fixes the problem.