If we're having a serious thread about this, my real answer is
Call Connected Thru The NSA. I used to be firmly in the
xkcd://479 camp (indeed, insisting on an appropriately electronic ring, rather than a recording of a mechanical bell), until I realised that I was annoying people (including myself) on trains by not recognising that it was my phone doing the ringing. I hardly ever get voice calls, but when I do, it's probably important.
I have a 33k6 modem handshake as an SMS notification. This started as a joke (you can tell how old people are by how they react to it), but I've kept it because the broad frequency spectrum means it's surprisingly good for hearing on a moving bicycle.
I have that bird that sounds like a rubbing disc brake for twitter, and an untitled goose honk for IRC. Some appropriate generic system noise for email.
While we're at it, the deaf alerting system I molished for barakta plays noises (yes, I know, but it's useful some of the time), and there's a directory full of wav files and appropriate mappings to events. Highlights include Rod Hull and his cast of drama students for the doorbell, the ST:TOS comms whistle for generic reminders, an appropriately submarineish PING! (one ping only) for manual pings, Talkie Toaster for anything relating to bread, the Red Dwarf food dispenser for anything relating to fish, a lemming saying "OhNo!" for power failure, Oscar the Grouch sings about how he loves trash when it's bin night, and Trimphone samples in appropriate cadences
[1] for the phone ringing. The fire alarm is, rather boringly redundantly, a sample of the fire alarm.
[1] I have asterisk programmed to do a Leftpondian single ring for internal calls, and a triple ring for calls from someone (after a database lookup on the caller ID) we might actually want to talk to.
