One of the best moments of BG2 for me was the time I first met a dragon -Thaxll'sslyllia the Shadow Dragon-.Its sheer size was frightening and when the battle began came the best part -the music.It was brilliantly composed; catching the mood of the battle and the horror of creature.So when I got the acm converter program I immediately thought converting the music to wave and then mp3.But to my surprise it was prepared in parts(like all other battle musics); to change according to the outcome of the battle I think(A cheaper way of Imuse;Lucas Arts' dynamic music system).How can I fix this problem and make a single wav from the mass?The music is in music\brd directory and is simply the best combat music of all BlackIsle games.

Just about any sound editing program lets you combine snippets like that, but only in wav format, of course.
Mp3 is meant as a final product, not something you go and edit and/or combine with others. So export to wav, find some decent sound recording program and you can start combining those wavs. (and export to mp3 later).
I'm sure you have an appropriate program in your Sound Blaster folder, but even windows Sound Recorder can probably do the trick.

A good editing program is Syntrillium's Cool Edit 2000. You can download an evaluation copy , at no charge, that will allow you to splice music together. http://www.syntrillium.com