There are several approaches on how you can debug such behaviour:
1. Observe the AudioSource components you're referencing, to see if any of them or their GameObjects get disabled at runtime;
2. - if all those actions are in one state, you can use the halving-technique by disabling/deleting half of all the actions, then run the game to see if the error still gets thrown; if yes: halve the remaining ones, if no: undo and disable the other half until you close in on the action(s) that are causing trouble (of course you shouldn't save the scene during debugging to easily roll back any changes)
- if you have those actions in several separate states you can just exclude certain states by not transitioning to them to see which of the remaining states throw errors and then narrow those down by halving their actions;
3. Use the FSM Log to see what actions and transitions the current FSM goes through or the FSM timeline to see, when which state/action gets called (both show only information at runtime and are located at the Menu Bar>PlayMaker>Editor Windows);
4. Check if there are any errors in the Error Check window (at the bottom left of the PlayMaker Editor, normally showing 'No Errors' with a green circle) that could give additional info on wrong action setup
But judging by the error you're receiving, my best bet would be that there is just some other action possibly from a different FSM disabling the targeted AudioSource(s); if so you'd need to find that through different means.