DEVART fixed the loud and long part with Calendar Alarm app, now available in the app store. The alarm syncs with the standard Calendar. There are 12 sounds to choose between and the sounds rings for 10 or 30 seconds. In the upcoming version the duration will be prolonged to 2 minutes and a repeat/sleep will be added. App id 367803799
While it's nice to have customizability (Calendar Alarm app), no one wants an extra step for calendar alerts. At this time, there's no way to change that awful calendar alert sound without a 3rd party workaround, like the Calendar Alarm app, or without jailbreaking.