According to Wikipedia A2DP enabled devices need to support the SBC codec only for audio transmission. MP3, AAC, ... can be supported but are not required to.
Which audio codecs are supported by Windows Phone 7's A2DP feature?
Searching the knowledgebase for
kbwp7 ad2p only yields one result which isn't really promising, so there is no information released about this. Since a lot only implement SBC, it's fair to assume Windows Phone didn't implement more than SBC either, simply because it doesn't need to implement it because it would be compatible with most mobile stuff using SBC.