How to change system sounds?

In your system preferences, select the Sound pane, and click on the Sound Effects tab. There you go!

Oh, and the Speech pane controls what voice you want for automated alerts.

 

You can't change the volume sound. It's built into the operating system and is unchangeable.

To change the alert sounds, you just select the one you want and exit System Preference.

[EDIT] After re-reading your post, I get the feeling that you want to assign a different sound for a specific event (i.e. when there's an error, X sound plays; and when the modem detects a ring, Y sound plays). I know you can do this in Windows, but I'm not aware of any way to do it in OS X. I think only one alert sound can be used system wide, and that sound is used for all events. [/EDIT]

 

this might do it, not sure. apple thinks different alert sounds are too confusing to the average user. the feature will probably turn up before too long...

pnw

 

Once you select a sound in preferences, it will play for system notifications. However, some apps, including mail, ical, and ichat, will allow you to assign a sound for an event (ie- new mail, event alert, new message, etc.). You can do so by accessing the preferences for the specific application. Once you have "tested" the sound, as long as you don't test another, that's it.

If you want to add personal sounds, convert mp3 or *.wav files to *.aiff with itunes. Just change the encoder from mp3 to aiff in preferences/importing. Then drag the file into /user/Library/Sounds. You can then point the system and your apps to the new sound. Enjoy.