This is going to be a bummer, but the very thing that helps us recover from a TKR--elevating toes above nose--can also cause or aggravate acid reflux (GERD). And you don't have to notice heartburn or sour taste to have GERD. The most common symptoms are hoarseness, a phlegmy/postnasal-drippy feeling and even some wheezing. Lying down can cause stomach acid to flow back into your esophagus and windpipe if your esophageal sphincter muscle is weak. (Especially if, like many of us here, yours truly included, you are overweight--more so the older you get). If you have any doubts, see an ENT, because it'll show up on your larynx before it does in your esophagus. There are medications and diet changes you can make--especially avoiding acid or pepsin-stimulating foods (at least w/in several hrs. of lying down) that'll allow you to heal your pipes and your knee. Check Dr. Oz' website. Meanwhile, if you are going to elevate your knee above the level of your heart, you're going to have to elevate it at a much steeper angle if you are also going to make sure your head and esophagus are elevated sufficiently to minimize or prevent reflux.
Another reason for a weakened voice is underuse, which prevents the vocal cords from touching along their entire length while vibrating--and if we are spending as much time at home resting as we need to we're probably not talking or singing as much as we did before our surgeries. Sing along with the TV or radio. Go online for some free YouTube vocal exercises. Use your lower register (chest voice) as much as possible, which bulks up and firms the vocal folds--don't strain, but strengthen the vocal folds. If you need to raise your voice to be heard, raise the volume and not the pitch. In other words, shout if you must (from the abdomen) but don't scream (from the throat).