There are multiple ways to do this, works best if you have WiFi as it eliminates the wired tether to the computer.
Option 1 requires the USB connection but no computer or WiFi
Option 2 requires a computer and WIFi but no usb. If you want to play content on the computer, see if the second option below works.
Option 3 requires WiFi but does not need a computer or USB connection
1)Plug a USB DAC into the iPad via the USB camera connection kit.
The iPad usb output is limited in the amount of power it can supply so a self powered DAC will work better.
This allows you to playback songs on the iPad itself
2)You can also use the iPad as a remote juke box controller for iTunes
This allows you to playback music on a computer, requires WiFi for connection
3)Airplay, this is built into the iPad, you will need WiFi and one of the following, Airplay Compatible speaker, AppleTV, Airport Express, or Airplay AV receiver (Onkyo, Denon, Marantz etc) that supports the Airplay protocol or
make an Airplay receiver using the PC + WMC but it is quite involved.
The Airport Express has spdif digital output that can be sent to an AV receiver/processor
This allows you to play the content on the iPad. You might be able to stream it from a shared folder on another machine although I have not tried it.
As you have a older iPad, check to see if you iPad can runs IOS v5 or later