I'm having trouble playing MIDI files from your site and from others. Can you help me?
(OR)
I just upgraded my system/browser/etc. Why do your MIDI files sound different now?

There are a number of possible causes that create these issues. My experience with this tells me that most likely something contained within this answer will help you understand or resolve the problem.

Have you recently upgraded your Internet Browser?
If the answer is no then check in either the site About link to see the current version or the Browser About Box (Help:About...) to see the copyright date. If it is more than two years old you are usually in need of an upgrade. There is often a logo in the browser's upper right corner that you can click on to go to a home site for that browser, then look for a "Downloads" or "Upgrade to..." link. WebTV browsers can be updated from WebTV.com.
If the answer is yes, then you likely need an upgrade to a Media Player. The two most prominent ones are Real Player and QuickTime or iTunes (both Apple), which are linked to from their name here. Each has a free plug-in that you can install into most any browser to play back MIDI, MP3, Movies, etc. Other browsers use WinAmp or the Windows Media plug-in. If your browser is not loading the MIDI files, which means that no MIDI or audio player with buttons is visible, one of these upgrades will likely work.

Is there a working player that appears to be playing the file but still no sound?
This may be an internal issue either with the operating system or the sound card. Locate the operating system volume control, which is usually available from a Control Panel in most systems, and represented by a speaker icon. There should be a Master level as well as individual levels for Input, CD, Wave, MIDI, etc. If you can hear System sounds and MP3s then check to see if the MIDI control is all the way down. If so then it needs to be raised. If you hear no System sounds or MP3s then check to see if the Master or Wave controls are all the way down. If this still does not work and the volume control on your speakers is turned up sufficiently then the problem may be with your sound card or the speaker connections, or even a power supply for your speakers. These are hardware issues that need to be resolved separately from the concerns listed here.

You installed a plug-in but now the music seems noisier or choppy
Most browser plug-ins for music interpret MIDI data into a streaming Wave file. MIDI sounds are derived from a table of sounds stored in your sound card, or are synthesized electronically. On older or less-expensive cards the results can be uneven or cheap sounding at best. The plug-in vendors have their own set of sounds contained in the software. As the MIDI data is processed through the plug-in each note is interpreted into a Wave sound by the plug-in creating streamed audio. This provides relatively consistent sound no matter where the plug-in is installed. However, some computers or sound cards don't have the processing speed or range to play the wave without interruptions or inherent noise levels. Your best bet in this case is to download the MIDI files to a folder in your computer, then play them back using the operating system's media player. Instructions should be contained with your operating system's help.

There is a burst of noise when starting a MIDI in QuickTime 6.x
This is a bug in QuickTime 6.x that causes this when using their internal sound set. The solution is to switch to the MIDI sound set built into your sound card. In QuickTime go to Edit > Preferences > Quick Time Preferences... Select Music from the dropdown menu. Select General MIDI in the list (which should de-select Quick-Time Music Synthesizer). Close the window, restart the browser, and the problem should have gone away.