faad always sets channel-positions on its caps, when nothing else does for the common mono and stereo cases.  When used in a pipeline containing an adder, this can lead to stuttering or skipping playback and other weirdness.

This gst-launch commandline (which is fairly similar in effect to the rhythmbox crossfading player pipeline) demonstrates the problem:

ADDER_CAPS=audio/x-raw-int,rate=44100,channels=2,width=16,depth=16,signed=true,endianness=1234

gst-launch-0.10 \
  adder name=a ! $ADDER_CAPS ! volume ! audioconvert ! audioresample ! queue ! tee ! queue ! audioconvert ! alsasink \
  \
  audiotestsrc wave=4 ! audioconvert ! $ADDER_CAPS ! a. \
  \
  filesrc location=file.m4a ! decodebin ! audioconvert ! audioresample ! $ADDER_CAPS ! queue ! volume ! a.

The section of the pipeline after the adder appears needlessly complex here, but it's there to allow us to plug in filters and additional sinks at runtime.

If you run that with -v, you can see that the caps of the elements downstream from the adder alternate between having channel-positions and not.  The exact effect of this on the output depends on the elements in that branch.  Sometimes it plays continuously but skips sections of the stream, sometimes it plays fragments with seconds of silence between them.