I downloaded a IFDG MD11 but it has no engine sound. You can hear flaps and landing gear moving, but nothing else
Can I add a sound file from another plane or is my file corrupted?
You can download another file if it's made for that particular aircraft, others will work at times but no guarantee.
The one you have , if you followed the read-me may be corrupted in some way.
Try installing it again.
if it doesnt come with a sound file then just copy the entire SOUND folder form your boeing 737 default, and paste it in your new aircraft folder