You will find the instructions for adding custom music as part of - Ancient Bioware PDF Tutorials. A few things have changed since that was released - first music can now either go in your -
..\Documents\Neverwinter Nights\music
folder or they can now be included in a hak file. Also, in EE, there is no need to convert your mp3 files to bmu. Instead you just change the file extension from mp3 to bmu. Finally here is the standard ambietmusic.2da (note the spelling, you have an extra unnecessary ‘a’ in your post) -
…just my 2 cents here: for the older Neverwinter Nights 1.69 edition there were some rules that had to be followed:
The mp3 had to be saved to 128kbs constant bit 44kHz stereo file format.
Maybe try to save the music again and see if it plays properly.