The difference between playing AudioCD/VideoDVD and playing mp3's is that the drive knows that the former are operations that require a continuous datastream and it is designed specifically to accomodate those needs when it knows it is reading such data. Audio CD and Video DVD are well defined formats that are designed to allow smooth reading of data. This is not true with an MP3 DVD or MP3 CD.
Your NEC DVD drive has no idea what an mp3 file is, and as far as it's concerned, it's just feeding chunks of files. It doesn't know that it's music and a slight delay in the reading of data will destroy the experience. When your mp3 playing software is in the middle of the song and requests the next chunk of data, the drive might have spun down or might have to refocus/reposition the laser on a different physical part of the disc. That's why there's a delay in reading the files.