The version of iCal in 10.5 has the ability to automatically add a default alarm to every new event -- enable this feature in the General section of iCal's preferences. However, there's no ability to choose the sound for these default alarms; you get what iCal chooses and have to manually change it each time. But there's a solution to that...

To change the default alarm sound in iCal, first quit iCal. Now double-click the com.apple.iCal.plist file in your user's Library » Preferences folder. When Property List Editor opens, click the arrow next to Root and select the Root entry. Click the New Child button in the toolbar. In the entry that was just created, enter the Item as Default alarm sound, leave Class set to String, and then type the name of the sound file you'd like to use in the Value field. Save preferences file and relaunch iCal. Create a new event, and you should see that its alarm is set to the sound you entered as the default.

[robg adds: Property List Editor comes with Apple's Xcode Developer Tools; if you haven't installed the tools, then the file will open in a text editor, but display gibberish. That's because it's a binary file. To edit it in a text editor, you'll need to convert it a couple times (before and after editing) as explained in this hint. To make things easier, you could use PListEdit Pro instead, which can open binary plist files ... or you can just install Xcode.

For the name of the sound file, enter it without the extension (Glass, Hero, Morse...) or path; just the root portion of the filename itself. iCal will look for sounds in both the /System » Library » Sounds folder and your user's Library » Sounds folder -- so you can store and use custom sounds in your user's Sounds folder. (I assume it would also find sounds in a /Library » Sounds folder, but I didn't test that.) Note that it seems, based on my testing, that sounds must be in AIFF format. I tried using MP3 files, and while they played once, iCal then crashed if I tried to change the alarm sound to something else. I didn't do extensive testing, but had the same result with two different MP3s. I converted those two files to AIFF, and they worked just fine.]