25.1 update sequence audio tracks 3 & 4 are silent (not muted and not panned)

Annyongly, 25.1 starts all new squences with the audio panned rather than balanced—I'm thinking I can adjust this in the preferences (?), but eitherway, this isnt the problem...

Nor are the problem tracks (usually 3 & 4) muted. 

Audio clips placed on tracks 3 & 4 register though the audio meters, but are not audible. The only solution I've found is to delete those tracks then the new tracks  3 & 4 work as expected. 

Is this a bug, or am I doing something wrong? 

Thank you!!