no audio or level metering (track or master) when playing a session or waveform.

My 30' session [radio programme] comprises 30 clips on 6 tracks, roughly. I get no audio or level metering (track or master) when playing a session. Same if I open one of the clips alone in Waveform. This is a session from a backup folder, created by Audition. Other sessions in that folder of this programme are the same, but sessions in the same folder for other programmes are normal. If I add a clip to one of the tracks, that plays [audio and level metering]. Muting and soloing tracks makes no difference. I also selected all clips, muted, then un-muted - no change. This has me stumped. Any ideas, please?