I am a Mac user, and a MIDI writer/listener/Editor as well, so I have had similar problems --Before we get to your problem, I'll mention that I save MIDIs and listen to them with SoundApp, which is a freeware program (you can get it from ZDNet, or with a search) which will play any kind of sound file and lets you set up a playlist, as well--Best thing is that you can mix the MIDIs with MP3s, WAVs, AIFF's, and listen--

But back to your problem--

The first thing that I would do is to close the browser, then go into    preferences in the Systems Folder control panels and move the Quicktime Preferences to the trash--

When you re-open , Quicktime will revert to default settings, and your MIDIs should play in the browser again--

If they don't, next stop is the control panel--
scroll down to Quicktime Settings--


when the box pops up, --go to "Plug-ins"

Scroll to "Audio",   in my OS, I click on the triangle to get a list of the different types of files,
Go down to MIDI, there is a box that allows you to select Quicktime Plug-In, Quicktime player, or ´Do not use Quicktimeµ--I·d choose Quicktime Plug-In--

If that doesn·t help, move from plug-ins to Music--

There should should be a box that tells you what the music synthesizer that you are using is--it should say Quicktime Music Synthesizer(which is the synthesizer module that comes with the OS)--

if it doesn't, push the button that says "Edit" or something like that

A window should appear that features a box that says name, which says "Quicktime Synthesizer" or something like it, and there should be a box with a pull down menu next to that which allows you to choose the synthesizer--there will also be a button labelled "default" that will restore the default settings--

If you don't have Quicktime Synthesizer, it means that somehow or other, you have lost the synthesizer module and you'll have to custom install a new one, either from your OS disc, or my downloading it from the Apple Website--

(the box may offer you some other options, like "General Synthesizer" or ´Roland D-10µ, but these simply allow you to connect to an outboard device)--

If none of this works, did you accidentally install a version of Quicktime that has limited or restricted functions, or do you have multiple versions of Quicktime that conflict with one another? It is also possible that you are running a version that was made to run on a later operating system than the one you have--