master volume muting on playback

Hi,

Suddenly when my audio is playing past a certain point (going from one audio track to another separate one on the same audio track line) the master volume drops to the far left and I cannot hear any audio. For me to hear anything I have to drag the master volume back up to 0.0DB but if I click space bar to stop or start the track again the same thing happens and the master volume drops to the far left and no audio can be heard effectively muting it.

PLEASE HELP. this is driving me wild as I can see no difference in setup from the audio segment playing a few seconds before on the time line to the next audio sitting along side from it (this is all on track line 1) yet this problem won't stop.

Tks in advance.