Adjusting The Sound's Key
You can change a sound's key by clicking the Sound Details Tab on the bottom of the window.  Each sound can be in one of the following modes: Adjust To Project Key, Adjust Pitch By, Transpose By (Virtual Instrument clips only) or Do Not Fix Pitch. (Audio clips only)  Select the drop down to change the mode to Adjust to Project Key.

Adjust To Project Key
If in this mode, Mixcraft will adjust the pitch of the sound based on the difference between the project's key and the sound's key.  For example, if the project key was F# and the sound's key was F, it would adjust the pitch of the sound up by one semitone so that it was in tune with F# instead of F.

However, you can have more than one key change in a project.  A sound in Adjust To Project Key mode will adjust to the correct # of half steps in order to play in the correct key based on the most recent key change.  To continue the example, if your sound was in F and you had two key changes to A and then G, it would correspond to shifting the pitch of your sound by four semitones up  to A and then by two semitones up to G. 

The software adjusts the pitch by the shortest distance between two keys.  For example, if the project key was G and the sound's key was F, it will adjust it by +2 semitones, instead of -10 semitones.

Changing The Key Of A Sound
Assuming that the key of the sound is correct, you can change the key of a sound by adding markers with key changes and/or by changing the project's key. You do not need to change the key of the sound on the Sound Details Tab, but, instead, you should change the project key or add key change markersIn short, if you want to change the key of a sound, change the project key!  Changing the key of a sound should only be done if the detected key is wrong!

Changing The Detected Key Of A Sound
You must be in Adjust To Project Key mode to change the detected key of a sound.  

If the sound's detected key is wrong, you can adjust it via the Detected Key drop down control.  The original detected key will show up with an * next to it, in case you need to restore the original detected key.