ScreenFlow User Guide
Media Capabilities — MPEG-1 Audio

Audio may not play

Some old types of MPEG movies (MPG, M1V) may not play audio in ScreenFlow.

This is a well documented issue with QuickTime (which ScreenFlow uses for playback of MPEG).

Workarounds

The work around is to convert the media into a different type (such as MPEG-4).

Try a codec helper such as Perian (http://www.perian.org), which helps QuickTime play a myriad of normally unsupported file formats.