Negative Inductance and Cascaded DC/DC Converter Instability
From your numbers it seems like the instantaneous draw when the audio output is at the maximum peak of the waveform is too much for converter 1.  This would explain why it works at higher frequencies, since converter 1 then sees more the average rather than peak instantaneous current draw.

You say converter 1 puts out 12V at 1A, which is 12W.  To put out 10W RMS audio power would mean the peaks of the audio waveforms draw twice that instantaneously.  This would overload converter 1 by almost 2x according to your description.  At 1kHz audio output, the overload happens for only about 250&micro;s at a time.  Since the average draw is OK, converter 1 doesn't fault.

This is just a guess, of course, but it is consistant with the information you provided.