The Google Clock app now lets you record your own audio to use as an alarm. This is rolling out to users on the existing 7.3 release via a server-side update. I think this is limited to Pixel users, or at least users with the Google Recorder app installed (so some custom ROMs count). Seems Clock sends the MediaStore.RECORD_SOUND intent targeting com.google.android.apps.recorder, the package name for Google Recorder, explicitly.
Google Clock 7.3 Changelog:
- Improved tablet support
- Bug fixes
Clock combines all of the functionality you need into one simple, beautiful package.
- Set alarms, add timers, and run a stopwatch
- Keep track of time around the world using the World Clock
- Set a bedtime schedule, listen to sleep sounds, and see your calendar
- Pair with Wear OS devices to bring your alarms and timers to your wrist