If you can afford to spend some money, you can buy special audio amplifier. This device amplifies the sound that is played on the speakers, so they sound much louder. Depending on the acoustic system to vary power of the amplifier. Be sure to test it immediately after purchase.
To amplify the volume of one or multiple tracks at once use audio editors. If you want to achieve a change in not only volume, but EQ certain tracks, use Adobe Audition or Sony Sound Forge - these editors offer broad functionality and the best quality.
To increase only the volume, but multiple tracks, use the program mp3gain. It can increase the volume of the sound from multiple tracks at once, but you must create a copy of a file when using it. This must be the case that after processing the file will sound with interference.
The simplest method of increasing volume is to maximize all frequencies EQ in the player you are using. This method only requires the EQ in the program you use for music playback. Drag all the frequency sliders at the top-most position and play the desired track in order to ensure the safety of euphony.