I found a dependency between "audio.c" and "hotplug.c" which can cause an error while compiling ML. You need to open the Makefile and set CONFIG_AUDIOMON to "n" to reproduce this problem. "audio.c" (or: the compiler) will then report an undefined reference to 'audio_monitoring_enabled' which is defined in "hotplug.c". But that file is not used in this scenario.
Issue #404 resolved