UPDATE: There is a number of apps that can nowadays record a ringtone or create it from your music files. One of them is Ringdroid.
However, if you already have a voice memo and you want to convert it into a custom ringtone, here’s what you do on HTC EVO 4G:
1. Open Voice Recorder
2. Find (or record) the desired memo
3. Press and hold the memo to get the interaction menu and select “Set As Ringtone” (“Rename” first from a number to a better name if you wish). That’s it!
– This sets the default ringtone so to use it only as a ringtone for a specific contact, go to Menu – Settings – Sound – Ringtone and re-select your default. Then a custom ringtone can be set for each contact.
– If you delete the voice memo, the ringtone will be gone too.
See this on YouTube: