My apologies, overlooked using a rule which definitely would allow you to accomplish what you want - sorry about that.
You can create a rule for a given recipient set the sound that's played when a message is received
Create a new rule and
#1 - Create the email account to which the messages are sent
#2 - Click on <Advanced Options>
#3 - Confirm that the account is the properly selected --> click on <Next>
#4 - Check the box for <Play a sound>
#5 - Click on <Sound> and select the file you want to play - if you click on <Next> before doing this, not an issue since you will be asked to do that when click on <Next>
----then proceed accordingly
