Mixmatrix returns random garbage pointers as request pads. Reproduce with: 
 
        mixmatrix = gst_element_factory_make ("mixmatrix", "mixmatrix"); 
        sinkpad = gst_element_get_request_pad (mixmatrix, "sink0"); 
        g_assert (GST_IS_PAD (sinkpad)); 
 
That's because the initial pads array is not initialised to NULL, so it just 
returns random garbage in the request pad function. 
 
Patch attached.  
 
There's more stuff that looks fishy to me, like: 
 
  mix->grpsize = 8; 
  mix->outsize = 1024; 
 
  // start with zero pads 
  mix->sinkpadalloc = mix->grpsize; 
  mix->srcpadalloc = mix->grpsize; 
 
but I couldn't be bothered to look into it further, as I found that I can't 
link the pads I get from deinterleave to the mixmatrix ones (the deinterleave 
pads have audio/x-raw-float caps with buffer-frames=(gint)0, and the mixmatrix 
request pads require buffer-frames >= 1). 
 
Cheers 
 -Tim