This is really only half a tip because there’s one small caveat…
Want to hear music when your Mac starts, without having to lift a finger? Open System Preferences, then select Users & Groups. Select your user account on the left, then click the Login Items tab.
Open a Finder window, browse to your Music folder (or your iTunes folder if iTunes manages your music), then locate an MP3 you like. Drag it to the list of login items in System Preferences. This will add it as an entry.
And that’s it! The next time your Mac starts and you reach your desktop, iTunes will open and start playing the song. Once the song’s finished it’ll resume whatever playlist it was playing when it last quit, or from wherever it was in your list of music if you weren’t using a playlist.
Combine this with the ability of Macs to switch themselves on at preset times (click the Schedule button within the Energy Saver pane of System Preferences) and you’ve got a Mac-sized alarm clock! Or how about a Windows-like startup-sound?
Okay. Now for the bad news. There seems to be a bug with OS X Mountain Lion where, if iTunes is already open when you quit and you set in the quit dialog box “Reopen windows when logging back in”, then the tune won’t start playing… until you quit iTunes! In fact, once you quit iTunes, iTunes will instantly start up again and begin playing the MP3. Weird. The solution is obvious — just don’t use the “Reopen windows” option.