I doubt the voltage is low enough to be the cause. Measure the drop across the 220 and 100 ohms resistors on the -ve supply. The current thru the 220 should be > than that thru the 100 for the zener to be doing anything. If they are the same then something is drawing too much current or the design was poor and / or line voltage is too low.
Have you checked the speaker? If that is OK put a sine wave in and scope it until you find the issue.