It sounds like an Acrobat bug/limitation. I think it should be reported in the Acrobat forums.
OMG! You are a genius!
I changed the character style to a dot leader (using actual dots) and adjusted the tracking to pull the dots closer together.
I now have a leader line that is visually indistinguishable from our previous underscore leader and, when sent to Acrobat Read Aloud, the leaders are Blessedly silent.
Thank you so much! I am eternally grateful.
You could also use an underline attribute and use any stroke or dots you have in the stroke panel defined, even if you name it Feet, Wow or some other easter egg.
Aha Willi! … and the Read Aloud will say: Feet!, Feet!, Feet! …
I prefer: May The Force Be With You!, May The Force Be With You!, May The Force Be With You!, … [ maybe with Darth Vader's voice! ]
[ Of course, it's a joke! ]
-- That's what I suggested in post#2! --