<p>I want to serve a lot of big files in a Plone site. By big files I mean around 5MB (music) and a lot of them. I've already do it straight to the ZODB, not a good idea. I'm running Plone 3.1.1 and Zope 2.10.6.</p>

As you may already know, the long-term solution for this is supposed to be the ZODB BLOB support.
Ticket 6805
is probably the most authorative source on this. Unfortunately, the milestone is set to 4.0, and running it in production on an older release is perhaps not a good solution.
There has, historically, existed a lot of Plone products for storing files externally, keeping only metadata in the ZODB. I have tried several of them, and from my experience, there is not a single one that works well with current Plone/Zope releases. Don't trust me on this, though, I have not tried any products of this type the last year or so.
Personally, I would go for a solution that is as simple as possible and doesn't involve Plone more than neccesary. Storing the music files on disk, serving them directly from apache/whatever web server you use, keeping only metadata in Plone - in a product you write yourself, will give you a robust solution with good performance. That is, your product should produce links to a path on your web server where the music files are available.
If you require authorization for download of the music files and assuming that you run lighthttpd or apache in front of your Zope, looking at a solution based on X-sendfile is probably the best option. With X-sendfile, you keep the files on disk, and add a header (X-sendfile) to the response when a music file should be sent to the client browser. The web server will pick this header up and send the file to the client, without Plone being involved.
Some pointers:
http://tn123.ath.cx/mod_xsendfile/
(The apache module)
http://john.guen.in/past/2007/4/17/send_files_faster_with_xsendfile/
(Ruby example)