help with resistance

[Cookie Monster]

so basically i've got a tiny pre-made amp with a speaker. It runs off two AAA batteries in series. it has a normal male jack (u no the one that fits in the psp) and i want to internalize it. im planning to give it power from the pobo but i cant figure out what resistance i need. And if i just put an LED in parallel with the speaker would it be sound reactive? so any ideas