Netscape, at least, will tell you what plug-in you need to get if you can't play or view something, and it usually will give you the option of downloading it--

SoundApp is a handy program to have,(you can download it free from ZDNet--Click here) because it will allow you to play almost any of the music files you might download (though it won't help with streaming audio). Also, FYI, any MIDI files that you might listen to on-line are stored in Netscape in the Netscape Preferences folder the cache folder, or, with Internet Explorer, in the Temporary Files folder in Explorer Preferences folder. You can just pull them out and keep them in a folder on your hard drive, and you can play them back with Quicktime or with SoundApp.

Also, the MIDI files can be opened in Music Notation programs as sheet music, and opened in a sequencer program where you can edit them and play them back on a synthesizer--