Can you cast a spell while falling?
So let's say I'm falling roughly 50ft and I have a Doss lute (instrument of the Bards) with which I want to cast fly so I can stop my fall. Would this work?
Xanathar's Guide to Everything says
The rule for falling assumes that a creature immediately drops the entire distance when it falls.
Using just the standard ruling, no matter how high the person was, they will have already fallen the entire distance before you or anybody else has a chance to cast fly on them (unless they had a readied action).
If you had a readied action to cast the spell on yourself in response to some perceivable trigger, then you would be able to do this however.
If you are higher than 500 feet in the air, Xanather's Guide to Everything provides an optional rule that would allow you a chance to do this, but even under this optional rule someone under 500 feet will not have a chance to cast fly on themselves.
Nothing about casting from the Doss Lute changes the time it takes to cast fly: 1 action. Unless you have a readied action then you are going to fall.