Here is a schematic diagram of a diy headphone amp circuit which will increase or amplify the audio in your headphones. You can use this headphone amplifier circuit with the audio devices which has less audio output in headphones and you want to increase it.
This headphone amp schematic is very simple and all parts can be easily found in the market. Transistor BC239C is doing preamplification of the incoming audio from your device and transistors BC337 and BC327 are amplifying the audio. Use 3v battery to power the circuit, all capacitors should be 10v ratings.