an LM386 op-amp (Many are available, including RadioShack part #276-1731 and Digi-Key part #LM386N-3-ND.)
a red LED
an 8-pin IC socket
an 8 ohm speaker
a 100 μF capacitor
a 1/4″ mono phone jack (a guitar jack)
a 10k ohm variable resistor (Variable resistors are also called potentiometers or pots, and in this case you want an audio taper; see Audio Tapers vs. Linear Tapers in Building It.)
a control knob cover, such as RadioShack part #274-415
a 470 ohm resistor (coded with yellow-purple-brown stripes)
a DPST toggle switch (This can be a little hard to find, but can easily be replaced with a DPDT switch like RadioShack part #275-666; it’s pricey and far beefier than the project calls for but is also easy to install and looks great. For a full discussion of toggle switches, see Types of Switches in Building It.)
a 9-volt battery clip
a 9-volt battery
24-gauge insulated wire, either stranded or solid-core
22- or 24-gauge bare bus wire (This is uninsulated solid-core wire; the 22-gauge is slightly thicker than the 24-gauge and thus preferable.)
whatever enclosure you choose (A cigar box is nice; hint hint.)