Options just off the top of my head. Not necessarily in order of likelihood:
Option 1) Turn the music up. And comment "need to turn the music up, having a hard time hearing it".
Option 2) On a track that has a call out/call back....get class to say it. "....come on, let me hear ya.....", so at least you break up their conversation and get them in to the group.
Option 3) Smile a sweet, innocent, full of expectation smile and say "....It's not so helpful to other participants to talk during the tracks...between tracks is fine....(or something like that)".
Option 4) Do none of the above during the class, but approach the individuals privately after class and explain that you could hear them talking during the tracks, which is very distracting to other participants. Sometimes, it is simply a matter of not thinking about it, and tolerating it during one class without making it an issue in that class but addressing it afterward can be effective.
Option 5) Be teacher like and simply say, just like you do when you're telling them to take out a riser, "We're going for the best workout we can get today, so come on and join in. Save the convo for later. Or at least, text it (need the smile at your blatant attempt to soften the words...but it actually does soften it if it's a real smile). Are you ready?.....Are you ready?!"
Option 6) Be a teacher or old time librarian, catch their eye during a track, and put your finger over your mouth, the "shssh" signal.
Option 7) Go big time ego on them (so over the top they know you're playing, but also can then get it) and say "hey, I have the mike, so the in track conversation has to wait", laugh, and get on with the class.
Option 8) Make them go stand in the corner for a classic time out (that's being a teacher on a bad day).
Option 9) Put on harder tracks, encourage them to push themselves, and get them breathing harder so they don't have interest in using that oxygen to talk. 