If this is a sound recording situation, the usual meaning of 'to rack it up' is 'to increase the level (volume) of the sound' — the implication would be that it is already fairly loud, and you need to make it louder still. 'crank it up' / 'crank up the volume' is also used.

Other interpretations are possible, though, depending on the wider context — for example, it would be very helpful if you just told us between whom this dialogue is taking place!