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.

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