The serial port hang is due to the player software, which for some peculiar reason likes to change the baud rate on the serial port when on DC power. You can add lines to config.ini to prevent it:

[serial]
car_rate=115200 ; Set's default rate when on DC to 115kbps

And again, for the last time, stick some printk()'s into the kernel, in hijack.c and in the various routines of empeg_mixer.c, so that you can observe the exact sequence of things at startup.

Cheers