Audio Length of File Upload as Attribute?

Is there a way to make the length of a piece of audio or video uploaded by a customer into an attribute we can charge against (like $2 per audio minute)? I have gotten this cart installed on my website and have also installed a "File Upload Required" plug-in to do this but I am not very experienced and I can't seem to find how to make this file property value a cart attribute i can charge against in the files. I see the attributes being changeable and making them upload-dependent, but i don't see how i could take the further step of pulling a property value (audio duration) of that file and using it to charge the person.

I don't necessarily need a strict walk-through but pointing me in the right direction would help me greatly. I haven't been able to find it here in the forums, but i could be searching for the wrong terms or something. Thank you! I would not want the customer to be able to change this amount freely, but I would want it to read the file length in terms of audio/video and multiply that times a price I specify. Sorry in advance if I am using incorrect terms.

Using newest zencart 1.5.3.

okay. As an update, let's assume that I can assign this audio duration to some html or js variable. Is there any place you can direct me to read as to how I can have the cart read the amount assigned to the variable and charge based on that?