Coexisting Exchange 2010 UM with Exchange 2007 UM -- Audio codecs

We have to coexist both Exchange UM 2007 and Exchange UM 2010 for a few months.  On 2007 UM we were limited to WMA codec for message encoding but now on UM 2010 I see we can use MP3.  If I select MP3 audio codec on Exchange UM 2010, how will that affect users who's mailboxes are still located on 2007?  Has anyone tried this?

Thanks!