Looks like ioctl(,EMPEG_MIXER_READ_SIGNAL_STRENGTH) to the /dev/mixer (DSP) device.
Code:
include/linux/empeg.h:#define EMPEG_MIXER_READ_SIGNAL_STRENGTH _IOR(EMPEG_MIXER_MAGIC, 18, int)
include/linux/empeg.h:#define EMPEG_MIXER_READ_SIGNAL_STEREO  _IOR(EMPEG_MIXER_MAGIC, 19, int)
include/linux/empeg.h:#define EMPEG_MIXER_READ_SIGNAL_NOISE   _IOR(EMPEG_MIXER_MAGIC, 21, int)
include/linux/empeg.h:#define EMPEG_MIXER_READ_SIGNAL_MULTIPATH _IOR(EMPEG_MIXER_MAGIC, 22, int)
include/linux/empeg.h:#define EMPEG_MIXER_READ_SIGNAL_STRENGTH_FAST _IOR(EMPEG_MIXER_MAGIC, 24, int)