|If your forum supports file attachments, you could just attach the audio file to your post.|
Otherwise you'll need to find a free audio file host or host it on your website. After you've got it hosted, either use your forum's audio-specific BBCode (if it has one) to link the file, or the basic URL BBCode and do the same. If your forum supports HTML code, using the anchor tag does the same thing.
<a href="http://www.mysite.com/audiofile.mp3">My audio file</a>
Apologies if I don't respond to your reply immediately. I don't check this site daily, but you're welcome to PM me as a reminder.