As far as I am aware, mp3 isn’t one of the supported formats for dvd audio. I think you’d need to convert them to pcm, mp2 or ac3. Don’t take that as gospel though, since I’ve never tried to add an mp3 into a dvd. I’m just pointing out that mp3 is not on the approved list of audio formats.
A good authoring program should be able to add these converted files and let you to make separate buttons on the menu to access them if that is what you intend. Which program are you using? DVDLab Pro can make what they call an Audio Title, which is a “movie” that has only audio, or just one still frame while the audio plays.