mkisofs $FILE | cdrecord -v fs=6m speed=4 dev=0,6,0
You are missing a small thingy: "-" for cdrecord to read from stdin.
It is the same as the example given by man cdrecord:
mkisofs ???R /master/tree | cdrecord speed=2 dev=2,0 tsize=XXX
to be precise:
quote from man cdrecord:
"
and then run
mkisofs -R /master/tree | cdrecord speed=2 dev=2,0 tsize=XXXs -
where XXX is replaced by the output of the previous run of mkisofs.
"
audio in data format does not have to be mp3, BTW :)
cheers, tobias.