I'm not sure how to change the speed if that is possible. I'm also not sure about bass/treble adjustments but have you tried a different voice to see if you can find one that works better without those changes? You can look under Settings then Hub Details.
I will often use the Media Device option rather than the Speech Device option since you then have the option to set the volume in case you last listened to something very quiet.
Thanks for the suggestion.
I actually changed it to Amy Female, British English and I found that was easier to understand than the default. (No, I'm not British.)
I saw that I had a choice of "music Player" or "Speech player and I chose "Speech".
I'll see if that makes a difference.