Ok,
After searching for a while, I found out where the problem comes from. It does not come from SoundManager.js. It comes from the PHP session files. In the project I'm working on, you have to be logged in to have access to an audio file with SoundManager. So when SoundManager loads the file, a PHP is run to check if the user is logged in. Thus mean that the session files are locked for as long as the file is not completely loaded. My other calls to the server are not able to complete because PHP can't access the session files.
I found this post which explains a lot: How do I configure Apache2 to allow multiple simultaneous connections from same IP address?
Thank you