This page shows a few ways to sense touch or force and convert it to an analog voltages.
1V/Oct analog keyboard interface
This is a vintage style analog keyboard interface that is used e.g. on the stylophone to convert touches to pitches in 1V/Oct. More sophisticated version with note priority and touch/release detection can be found on music from outer space.
Velostat Pressure Sensor / Moisture-to-Voltage converter
This circuit can be used to detect finger pressure or any force on a surface with the help of a pressure sensitive sheet material called 'velotstat'. You can find a tutorial how to build the physical sensor here.
The same circuit can be used with a different type of physical interfaces as well. Instead of the velostat material as a variable resistor we can use a layout of wires known as a 'moisture sensor'. We can hookup our moisture sensor to the circuit above instead of the velostat sandwiched between two electrodes. Here is a video showing how to build a moisture sensor. Ciat Lonbarde's Tocante instrument might use this simple technique to sense touch.
By replacing the velostat or moisture sensor with a Light Dependent Resistor (LDR) you can use the same circuit to detect ambient light variations.
To convert the frequency of a musical signal to a voltage, we can use this relative simple frequency detector circuit. Note that the output is linear in frequency, so it won't match the 1V/oct input of an analog synthesizer. That means pitches will be out of tune. Their might be ways to remedy this by using a logarithmic amplifier instead of the linear amplifier in the schematic. This is shown by the next schematic.
You can use an envelope follower or envelope detector to track the loudness of a signal and use that to control e.g. the loudness of a synthesizer.