Take a mono channel wav for example and try to reencode it in wav using
audioconvert beteween...

like :gst-launch-0.8 -v   filesrc location=test-mono.wav ! wavparse ! 
audioconvert ! wavenc ! filesink location="test.out"

This will produce a 2 channel wav file 

/pipeline0/audioconvert0.sink: caps = audio/x-raw-int, endianness=(int)1234,
width=(int)8, depth=(int)8, signed=(boolean)false, rate=(int)8000, channels=(int)1
/pipeline0/audioconvert0.src: caps = audio/x-raw-int, rate=(int)8000,
channels=(int)2, endianness=(int)1234, width=(int)16, depth=(int)16,
signed=(boolean)true

When it should be 1 channel

The issue is in gst_audio_convert_fixate (GstPad * pad, const GstCaps * caps)

  if (!GST_PAD_IS_NEGOTIATING (otherpad)) {
   try.channels = 2;
   try.width = 16;
   try.depth = 16;
   try.endianness = G_BYTE_ORDER;
  } 

I have no idea why it return these defaults... to me it seems it should return
null , and if it does return null it fixes the problem...

Anyone would know why there was these default values there ??

if not : if (!GST_PAD_IS_NEGOTIATING (otherpad))
            return NULL; 

would fix it....

Thanks a lot

Antoine Tremblay