If the original disc has mp3's on it, it's not the same as a true Audio CD.
There's no such thing as an Audio DVD in the sense that it'll play exactly like an Audio CD would in your standalone CD player.
The Audio DVD's that do exist are a totally different format and you need special authoring packages to make them.
What you need to do (if you really want a true CD-DA Audio CD) is to split the songs up onto multiple CD's.
With MP3 being a compress format, you can fit loads on a CD but to play them back you need a player that supports MP3. Those types of discs are just burnt as data discs in Build mode.