Is there a program that will play audio where you can choose to play either treble or base separately or together?
Using an audio editor like Audacity (or any fully fledged DAW, for that matter, but I strongly recommend Audacity if you don't have any audio software to begin with) you can use a filter plugin to filter any frequency component you choose to. You can create two different tracks, apply different filtering to each and select to mute each track at will.
NB: as Lawrence Payne notes in his answer, this will not completely separate the right and left hand parts of the piano piece, as some high frequency harmonics of the lower pitch notes' spectrum will still be heard on the "left hand" channel. Also there may not be a complete separation of hands with a fixed note range for each note.
Going a different way than working with audio, depending on the piece, it may be possible to find a (or several alternative) MIDI rendition(s). Do a web search for "MIDI <"name of the piece">".
Depending on how the particular MIDI rendition is made, different instruments, or left and right hand for piano, may be in different MIDI channels, in which case most MIDI players will allow to select or mute different channels (the best free standalone MIDI player I know is vanBasco's Karaoke Player).
If it is a piano piece were both hands are in the same MIDI channel it's a bit more complex, but it's still possible to load the MIDI into a MIDI editor of some kind and separate the note events for the two hands into different channels (may be a more or less simple or complex task depending on how the note ranges for the two hands are more clearly separated or not). If you need to go that route place a comment here and I'll try to give some guidelines on how to do this.
Sort of. A multi-band e.q can cut the highs, boost the lows, or vice versa. But low notes still have high frequency content, and taking out all of the top will reduce their clarity.
I recommend a program called Transcribe! https://www.seventhstring.com/xscribe/overview.html
It will filter frequency bands, loop sections of the music, slow down playback etc. Nothing you won't find in many other audio players, but all bundled in a convenient package and not expensive.
I have no idea how accessible it is to blind users. Sorry.