Did you have headphones on? I've had my CTX lose the external port to the headphones (silence) and then kick back in after several minutes. The screen will still be working. Restarting the CTX doesn't matter - silence, until the audio returns (usually with a loud overshoot and then back to threshold). I've tried submerging the coil, control box, and GG headphones and unplugging and replugging the audio plug. It seems that when I dunk the headphones, they kick back on (but it may be a coincidence). I'm probably the only one experiencing this problem.
The other thing that happens is iron in the soil nulls the threshold - the audio goes dead - but the screen keeps on kicking. The null can be repeating at intervals in highly mineralized soil - turning off the audio every 30-45 seconds and lasting for 20-30 seconds, or so. During the null you can lay the coil on a metal scoop and not get any response, then the audio kicks back on suddenly. Rather irritating.
Of course, if there is DISC'd areas set, you aren't supposed to get an audio tone in those zones. But I figured that wasn't your problem. Generally, if you use a lot of DISC the detector loses sensitivity to weak (deep) signals - which may be your issue. Next time switch to zero DISC and see if you can mitigate the problem. For instance, if you use DISC and get a hit in an adjacent open area, you still may not get any audio but you will get a TID. Theoretically it isn't supposed to work that way, but it still can.