I get the exact opposite. Someone will be talking and to give an indication that I’m listening I’ll say something bland like “Oh,really?” and the person will repeat what I just heard perfectly well. I have never bothered asking what the speaker thought I said that prompted the repetition.
You should try these ones:
“Speak the fuck up, dear”.
“Speak the fuck up, mom.”
“Speak the fuck up, officer.”
“Speak the fuck up, judge/your honor”.
All bound to get immediate responses. Maybe not the ones you intended. 
Whenever you don’t hear what someone has said, just reply
“Stanley Baker, I think it was”
This has the excellent effect of stopping the mumbler dead in his tracks and saying
“What did you just say? I asked you what time the train was”
Confuse the buggers - that’s my advice.
Mmmemru muble, a mummer mmemoble nabbuffble bumbbermmen?
“I am sorry” In britain is accepted to mean “I couldn’t hear you”
Actually saying “I can’t hear you” is slightly impolite (only slightly)
WHAT?! HUH? SAY AGAIN!
My favorite is this:
Why in heaven do you assume that I heard everything you said except the last two words? If I ask you to repeat yourself, repeat yourself, dammit! You’re not a canyon, from which echoes only the final part of a phrase; you can repeat the whole thing.
Daniel