"an autoharp" is too generic to easily answer. An old black one? A more heavy-duty Oscar Schmidt?

Major concern IF it is an older harp...be SURE you do not use strings heavier than the original! Just a few too heavy strings can cause warping & distortion!

The major 'sound' concerns has always been with the first 2-3 unwound strings (the A#, B and C). I sometime replace those with wound guitar strings. (This is on a 21 bar Schmidt of modern construction). This make those 3 more easily tunable, although they will not last 'as long' relatively.

As to felt... *IF* you can obtain something other than the fairly soft white felt offered in pre-cut strips, it is nice. I once got some from a scrap bin at a gasket company..(they had an entire catalogue, but required ordering entire rolls). I got about a foot square of gray felt that was thicker and more resiliant and has lasted for many years with no appreciable wear...[it also makes less distance for the chord bars to travel when pressing down, allowing less work and faster chord changes....but required 'shaving' on the bass strings because OF the thickness.]

Not knowing where you live, I have no idea how easy it might be to get felt. I have never needed to repeat MY search.