Using an Android device as a remotely-controllable music player.
Well, perhaps you can play Bach on a piano, if you have the skill and the patience; but you can't hear Bach's music the way he and his contemporaries would have heard it because, in general, we don't using the same tuning schemes. This is an article about the mathematics of music tuning, from Pythagoras to the modern equally-tempered scale.
Although the Java Media Framework can do some very sophisticated things, it remains relatively difficult to generate simple musical tones using Java. The de-facto method for specifying musical notes remains the MIDI file, although there are now more sophisticated approaches. It's surprisingly difficult to generate even a simple MIDI file in Java, not least because of the lack of unsigned data types. I wrote this article years (perhaps decades) ago, but it's recently become popular again, because of the use of Java in Android, and the lack of any Java API in Android for playing musical tones, other than by reading a MIDI file.
Animated musical scores are quite common on YouTube, for educational and promotional purposes. This article explains how to create one, using OpenShot and other Linux tools.
In many cases, pitch is just the musician's way of saying frequency. However, pitch has a perceptual element to it, and the ear can be fooled. This article explains one of the ways in which this can happen, and why it isn't correct to use the terms pitch and frequency interchangeably in music.
Audio CDs were recorded using a 44.1 kHz sample rate that is found almost nowhere else. If we have to resample this audio to suit more modern equipment, how much loss of audio quality will there be?