Missing Safari-plugin for audio/MIDI
Support for playing .mid files in Safari was removed in 10.8. The QuickTime plugin from 10.7 or 10.6 still works in 10.9 though. See https://discussions.apple.com/message/23948316#23948316.


Copy /Library/Internet Plug-Ins/QuickTime Plugin.plugin from a 10.7 or 10.6 installation to ~/Library/Internet Plug-Ins/ on your 10.9 or 10.8 installation.
Run these commands:

f=~/Library/Internet\ Plug-Ins/QuickTime\ Plugin.plugin/Contents/Info.plist
defaults write "$f" CFBundleName 'QuickTime Plugin.plugin2'
defaults write "$f" CFBundleIdentifier 'com.apple.QuickTime Plugin.plugin2'
defaults write "$f" WebPluginMIMETypesFilename com.apple.quicktime.plugin2.preferences.plist

Save this file as ~/Library/Preferences/com.apple.quicktime.plugin2.preferences.plist:

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;
&lt;plist version="1.0"&gt;
&lt;dict&gt;
    &lt;key&gt;WebPluginLocalizationName&lt;/key&gt;
    &lt;string&gt;en_US&lt;/string&gt;
    &lt;key&gt;WebPluginMIMETypeFilteringVersion&lt;/key&gt;
    &lt;integer&gt;1&lt;/integer&gt;
    &lt;key&gt;WebPluginMIMETypes&lt;/key&gt;
    &lt;dict&gt;
     &lt;key&gt;audio/mid&lt;/key&gt;
     &lt;dict&gt;
         &lt;key&gt;WebPluginExtensions&lt;/key&gt;
         &lt;array&gt;
          &lt;string&gt;mid&lt;/string&gt;
          &lt;string&gt;midi&lt;/string&gt;
          &lt;string&gt;smf&lt;/string&gt;
          &lt;string&gt;kar&lt;/string&gt;
         &lt;/array&gt;
         &lt;key&gt;WebPluginTypeDescription&lt;/key&gt;
         &lt;string&gt;MIDI&lt;/string&gt;
         &lt;key&gt;WebPluginTypeEnabled&lt;/key&gt;
         &lt;true/&gt;
     &lt;/dict&gt;
     &lt;key&gt;audio/midi&lt;/key&gt;
     &lt;dict&gt;
         &lt;key&gt;WebPluginExtensions&lt;/key&gt;
         &lt;array&gt;
          &lt;string&gt;mid&lt;/string&gt;
          &lt;string&gt;midi&lt;/string&gt;
          &lt;string&gt;smf&lt;/string&gt;
          &lt;string&gt;kar&lt;/string&gt;
         &lt;/array&gt;
         &lt;key&gt;WebPluginTypeDescription&lt;/key&gt;
         &lt;string&gt;MIDI&lt;/string&gt;
         &lt;key&gt;WebPluginTypeEnabled&lt;/key&gt;
         &lt;true/&gt;
     &lt;/dict&gt;
    &lt;/dict&gt;
&lt;/dict&gt;
&lt;/plist&gt;

Quit and reopen Safari.


After that you should be able to play .mid files in Safari, but you have to allow websites to use the plugin manually:



You can also play .mid files with VLC or QuickTime Player 7.