While I concur with all the others who recommend not doing this, there are some (few) products where start-up sounds makes sense:

1. Hardware boot, where [the Mac startup chime][1] and [the IBM POST beep codes][2], both of which signify that nothing has gone wrong with the hardware (or, alternatively, that [something _has_ gone wrong with the hardware][3]). Depending on the hardware issue, the audible alert may be the only indication of the problem. In the days before Mac OS X (when rebooting a Mac was a fairly common occurrence), the startup chime also solved [another, somewhat novel purpose][4]:
> Turning the Mac on is one thing, but being forced to reboot from a
> crash is a totally different experience. I wanted to avoid a sound
> that would be associated with the crash. I wanted it to sound more
> like a palette cleanser.

2. The Operating System (especially 3+ years ago), which took so long to start up that the chime notified people that it was worth bothering to come back to the computer from, e.g., making a coffee. [Microsoft commissioned Brian Eno to create the Windows 95 startup sound][5]:
> …he received a brief from Microsoft with “about 150 adjectives” to describe
> the desired noise. “The piece of music should be inspirational, sexy,
> driving, provocative, nostalgic, sentimental…” said Eno.

    [They changed the sound in Windows Vista][6] (the same sound carried over to Windows 7):
> Ball is the first to admit that the percussive beeps in past Windows versions could be jarring enough to bother nearby workers or interrupt others in a meeting. With the number of intrusive sounds from cell phones, handheld devices and other gadgets only increasing, that's something Ball and his colleagues were keen to avoid with Vista.

    If you're keen, [here's a blog with all the Windows Startup Sounds from 3.1 to 7][7].

3. An application which automatically connects to the Internet, to notify the user that it has done so in the background (e.g. instant messaging apps).


  [1]: http://en.wikipedia.org/wiki/Macintosh_startup#Startup_chime
  [2]: http://en.wikipedia.org/wiki/Power-on_self-test#Original_IBM_POST_beep_codes
  [3]: http://en.wikipedia.org/wiki/Chimes_of_Death#Chimes_of_Death
  [4]: http://musicthing.blogspot.com.au/2005/05/tiny-music-makers-pt-4-mac-startup.html
  [5]: http://www.electricpig.co.uk/2011/11/23/brian-eno-spills-windows-start-up-sound-secrets/index.html
  [6]: http://www.msnbc.msn.com/id/15656246/ns/technology_and_science-tech_and_gadgets/t/start-me-sound-vista/
  [7]: http://mashable.com/2012/10/24/windows-startup-sounds/