The only currency throughout the former Empire (and some places beyond) is Loud-Quiet Money. LQM acts as an IOU: the payer whispers the sound, and depending on the pureness of the sound, there is more or less of a guarantee that they will be able to assist the person. Thus, the upper class elite of the former Empire have the purest training, and can make the purest sound, and thus have the most chance of having a purchase accepted. A lower class individual, without training, will be unable to produce a pure sound, and thus the chances of their purchase being accepted is less.
While LQM is the only currency, it is not often used. Instead, barter is the main form of trade.