Mac mini built in speakers do not play all of sound I've a rather odd issue I've noticed while playing this song on Youtube:

When using the built-in speakers I only hear the background singing (a bit hard to describe: sometimes I hear almost nothing, sometimes the high-pitched choir, at some parts mixed with lower voices). However the moment I plug in earphones they play it just like it is supposed to. The same happens in both Chrome and Safari with both HTLM5 and Flash players, even when playing the Youtube stream in VLC, heck even when downloading the song and converting it to low-quality MP3: same result. I've never noticed that before on any other music file nor can I reproduce it with another song. Is there something left, that I might have overseen?

Version information: Mac OS X 10.7.2 on the new Mac mini 2.5 GHz i5
it a little bit odd, but the Sound preferences in System Preferences only shows the controls for the currently selected device.  If you plug in headphones for example, it only allows you select and alter the sound preferences for the headphones (and doesn't even show the internal speakers)

It maintains different sets of preferences for each output type.

Unplug your headphones, then open the Sound preference pane and see if there is anything amiss in the sound output tab.

(also please let us know if you have any software like Hear or Audio Hijack Pro that might be interfering with your audio output but thats a long shot)