Well, that was unexpected... ZuneMusic !?@#$
If you brave about editing the registry, you can try seeing if this key is the cause...
And if you're not comfortable, don't proceed (you've been warned).
Select the key in the left panel, right-click, select 'Export', and give it a name.
This will save a {yourfilename}.reg file that you can double-click to restore the setting in case something goes wrong.
Then try double-clicking the entry on the right-panel and typing another existing BI5 sound file name over 'alarm' (like 'gesture').