Is there any point doing 2 passes with CBR audio encoding with WMA Pro 10?

[Furiousflea]

As title really, thanks. I'm backing up my HD DVDs to DVD9 in VC1 format for playback on the Xbox 360 and encoding the audio as a 5.1 using WMA 10 Pro codec, if film is long then using 640k 48khz 24bit CBR otherwise 768k.

Need to know if I'll gain any quality doing 2 passes, since It's CBR I would assume that it wouldn't?

Thanks in advance for any info, Rob