"there must be a reason for it, nothing these guys seem to do is without some sort of symbolism."
Assuming they use these tapes not just for rallying the masses but to send signals (not an unreasonable assumption), it is more likely they will use static visual cues rather than audio cues because they cannot control how al-Jazeera will edit the tape.
As long as a single frame of the video is aired, even if it is a still-shot, the signal is communicated.
Laura Mansfield mentioned something about the positioning of the gun... I can see in several past tapes, it is turned towards one side or another, but always in the same position otherwise.
It looks like the Bali tape was audio only? I wanted to check back against images from the previous tapes that followed attacks.