As the Visualizer documentation says:
It is not an audio recording interface and only returns partial and low quality audio content.
and
Two types of representation of audio content can be captured:
- Waveform data: consecutive 8-bit (unsigned) mono samples by using the getWaveForm(byte[]) method
- Frequency data: 8-bit magnitude FFT by using the getFft(byte[]) method
Without modifying Android itself so that you can get hold of e.g. the Visualizer's input buffer I don't see how you could get data with a higher sampling rate / higher number of channels.