I give up. Who?
Okay, but only 'coz you made such a valiant effort to guess!
See attached - Brian's contribution, it seems, amounts to nothing more than the count-in on the last track of David Crosby's
For Free - but you sure can recognise his voice!!
[actually, it sounds like it may have been lifted from the
Pet Sounds bonus track "Unreleased Backgrounds" first released 1990].