Google is updating the Clock app on Pixel devices to make it much easier to record and use your own sounds for alarms and timers.
As XDA Developer reports, version 7.3 of the Clock app is currently being updated to include the new recording functionality. You'll know your device has received the (server-side) update because a new "Record new" option will be available in the Alarm sound and Timer sound settings.
If you want to use your own sound, tap "Record new" and the Recorder app will appear, ready to accept your audio. Once recorded, it can then be set as the sound for your alarms and timers.
The Recorder app is exclusive to Pixel devices, so this isn't a feature we should expect to see on other Android smartphones and tablets. Of course, it may prove popular enough that Google decides to offer similar functionality more generally.