I have a bunch of files in other formats other then mp3, i would like
to convert them to mp3, what is the best/most efficient use of disk
space.

For example I have things in wave files, what is the best mp3
conversion?

I have audio books (narrated books), what is the best conversion to
mp3?

I have 'audible' files, what is the best conversion to mp3?

I know the better 'bit-rate' is higher quality, but there is a saying
on a porch driving on a freeway, it can only go so fast. If my base
file is wave, or a narrated book, I don't need or think that 320
bit-rate is useful except burning HD space.

Currently I convert everything into 44100hx(i think) 320 stereo MP3...

Thanks for any advice upfront.
EK