Even tough the sound visualizer has `channels` as one of its parameters it wasn't using it properly. The biggest problem is that at each frame the index was being advanced per channel frame_count increments.
The number of channels also determines how many graph will be needed to display the graphs of the visualized sound files. Besides these two problems there were many others like incorrect playback time, cracking audio, etc.  which will not be mentioned.
To sample the signal a channel sampler was created based on the one used previously that returns as many sampled signals as there are channels.
This PR aims hopefully at fixing all the problems encountered, and it has been tested extensively using `Audacity` exported samples to ensure the visualizer fidelity on playback and graph appearance.

The sliding bar and the play controls were being drawn over each other.
Added milliseconds to the timer. The end of the sliding bar was off by one.
Changes to make code more efficient and better formatted.

To fix the problem I included the last two rows to the subplot block so that ImPlot can draw them each on its own line.