Cristina, we do hear a harmonica (although we don't see Deanna playing it), and yes, the item in her hand looks like it could be a harmonica. But it wasn't in her hand when she was dancing with Merle, and when she flopped on the floor. Then it suddenly materializes when she gets lifted off the floor. Where did it come from? And why would Deanna have a harmonica? This is a case where 2+2 doesn't necessarily equal 4.
Another guess: In a deleted scene, we see Fredo and Deanna show up for the party in Fredo's '56 Mercedes 300SL Gullwing coupe (he must have been doing very well in the Mickey Mouse nightclub business!). They're in the midst of a big fight, and Deanna throws something at Fredo, that he catches. I think it was her clutch bag pocketbook. The item in her hand later on the dance floor looked a bit thin for a clutch bag. But, if it was a bag, and Fredo'd been holding it for her ever since they'd arrived (a neat symbol of Fredo's hen-pecked relationship with Deanna), then it'd be more logical for him to hand it to her when he tried to march her off the dance floor, than for her to suddenly have made a harmonica materialize.