The short answer is of course they do. Otherwise why would every physical therapist on the planet use ultrasound machines to treat ankle sprains? Ultrasounds work on ankle sprains because they soothe the soft tissue surrounding the area. Essentially an ankle sprain pain is resulted from tears in the ligament and cartilage around the ankle. Specifically it is the Anterior Talo-fibular ligament that tends to be most effected with an ankle sprain. Most ankle sprains tend to be graded through grades 1,2 and 3. To be honest you should not have to progress to ultrasound for a grade 1 sprain.
However, with sprains like grade 2 or 3 that tend to linger for awhile ultrasound treatments may be necessary. Now to be honest you do not need a physical therapist for these treatments and you can do them yourself. Additionally, ultrasound machines really are not that expensive, costing $50. Heck,if you get one you might be so impressed with how it works on your ankle you might want to try them on shoulder injuries. Heck, let a friend use it. Here is a video on how easy it is to use:
https://www.youtube.com/watch?v=i8ms0pBPHZ0
Now if you look into purchasing an ultrasound do not purchase the $300 units. Those are used for imaging and you do not need that for Therapeutic ultrasounds. All you need is a simple basic unit. Amazon has one here that only retails for $90:
Now there is another company, Discount Medical Supplies, (see the link below) that currently has a model called the Current Solutions Ultrasound which they are retailing for just 50 bucks which is really all you would need for an ankle sprain. Click on the link below and search.
My point is you do not need a physical therapist to provide you ultrasound for an ankle sprain. It really is just as easy to do it yourself.