First, you have to set up your PC. Since a visitors to your webpage may use Netscape Navigator or Internet Explore, you should have both browsers. You also need plug-ins for these browsers such as YAMAHA MidPlug.

Web Browser  Netscape Navigator Internet Explorer Software Synthesizer YAMAHA MidPlug ROLAND Virtual Sound Canvas

Find your favorite midi file on the internet. Select the one that is not too large.

Site that provides freeware MIDI Music Laboratory

First, you should upload the MIDI file to your web site. Then you re-write HTML codes of your webpage. Add a command to play the MIDI file into HTML codes of the page that you want to attach background music to and upload it. You should write different commands for different web browsers. Netscape uses "EMBED" tag while Internet Explorer uses "BGSOUND" tag. If you use "EMBED" tag, it may be valid for both browsers. If you want to have the background music repeat, you should set TRUE for both "LOOP" and "REPEAT." If you want to be perfect, you can use JAVA script to distinguish which browser is in use.

Site that explains how to attach MIDI to webpages Miningco.com

You should test what you have done. You should use both Netscape and Internet Explorer when you test your webpage.