Step 1: Download YATA+
Step 2: Use the wav to BCSTM converter to convert your file.
Step 3: Save the file as bgm.bcstm and place it in your theme folder.
Step 4: Switch to blank theme (default works)
Step 5: Reinstall your theme.
Step 6: Profit.
No-one is gonna do your work for you. Not without a reason. This is something that you can learn from various guides on GBATemp.