Playing music and recording audio through headphones

Hi. I'm a hypnotherapist and currently use a purpose built box which hypnotherapists often use. I have an iPad and I can use garageband to play background music and record my voice over the top at the same time.

I have a headphone splitter and of course one jack is used for the clients headphones so there's only one other jack left for my headphone set which has actually has two ends on it (one for the microphone and one for the headphones). So although the iPad is recording my voice it's not sending it as an output back through the clients headphones.

Anyone know of a way round this? Or an app?

Thanks