Boost an audio signal for analog read - Envelope Detector
it sounds like what you are wanting to do is to detect a minimum threshold of sound so that your speaker power source can be turned on when an audio signal is playing. 

You need to amplify your signal while also avoiding voltages that exceed the maximum input specification and also don't go negative. One way to accomplish this is with an operational amplifier (op-amp) circuit. A specific type will accomplish all three requirements. An op-amp that is powered by a single-sided supply, for example +5V and ground, will prevent negative voltages and limit the maximum voltage exceeding what the Arduino can take. 

For your application, you don't really care about making a high-quality audio amplifier. You care about not distorting the signal (music) that you are monitoring and detecting a threshold level. Op-amps are good because they have high impedance inputs (they don't draw much (or any) current from your music signal) and they work with few external components. 

A classic text for learning about electronics is Horowitz and Hill. Alternatively, you can find references for circuit designs on-line with appropriate searches. For any op-amp that you buy, you can get a datasheet that tells you how to use it. The following reference goes into detail (maybe too much) about single supply op-amps. 

Circuits (and search terms) that you might find useful are op-amp follower, single supply, bias, and capacitive coupling. You'll need to purchase components. A good supplier is DigiKey. DigiKey has an on-line catalog that allows you to do a parametric search to identify the components that you need. You'll need a breadboard, resistors, capacitors, and the op-amp in a dual-inline package (DIP). 

One search filters the list of possibilities from tens of thousands to a reasonable number. The LM833 Datasheet here is a reasonable choice. 

Test equipment, like a single generator and oscilloscope can be very helpful when building amplifiers. You might be able to digitize your amplified signal with your Arduino if you don't have access to test equipment.