Here's something weird! Melodyne detected tempo at half speed

Not unusual at all, unfortunately. I and others have reported that Melodyne often detects tempo at half the correct value. If you've upgraded from the bundled 'Essential' version, its my understanding that you can edit the tempo map (or the detection criteria) to fix this in Melodyne. If not, your only option for proportionally scaling the tempo map to double all the values in SONAR would be this:
 
1. Select a MIDI or audio clip that runs the length of the project.
2. Go to Process > Fit to Time.
3. Convert the current Thru time to total frames (usually 30 frames/sec).
4. Divide that by 2.
5. Round and convert that new value to minutes:seconds:frames.
6. Enter that as the new Thru, and select Modify by Changing: Tempo Map.
7. Click OK.