If the audio warning occurred only once and did not reoccur since then, I would not be too concerned as the computer deemed not serious and there arent any fault codes recorded to continue.
However if the symptom occurs at predictable timings, try connecting OBD2 scanner before you begin to drive and see if the scanner can read the live data and possibly the code at the time of occurance as the fault code may be overwritten as you continue to drive.
Scan for codes. Also turn the key to the RUN position and make sure the check engine light actually illuminates.
My Cali will chime for no reason once in a very great while but if yours does it with any frequency then you could possibly have a bad ground or other minor wiring issue somewhere that triggers the chime. Could also be some other sort of very intermittent problem that comes and goes.