You can control the volume of the audio input in any of those screens, using the fader on the right.
There are controls for Mute and Mono as well, and an output VU meter to evaluate the volume of the incoming signal.
You have to be careful when setting up your volume. Remember that TrainYourEars applies a random equalization to the signal. If it applies a boost, it will raise the volume several dBs and it could clip the output.