[7.2] Can I use inbuilt loop points in external sound files?

No. While some sound editing applications allow you to build loop points into AIFF files (Macromedia's own SoundEdit 16 being a prime example), Director cannot recognize these markers and will just play the sound straight through.

[ed: This wil be included in a future version of Director 5+ and was shown at the 1996 Macromedia UCON Sneak Peaks ]