To get a Raspberry Pi to display a VU meter that looks like this:

Using an Alesis LineLink lead (or similar) as the input.

• Having booted with the LineLink plugged in, in Terminal type: lsusb
  You should see an entry for it.
• In Terminal type: alsamixer
  Press F6 to see all devices, select 'Linelink' or 'USB PnP Audio Device', then press F4 for Capture and use the up/down keys to set the level.
• Escape back to the command prompt and type: sudo apt-get install python-pyaudio
  This will install pyaudio for Python 2, not Python 3 so we'll use Python 2 for the VU meter.
• Download this file, save it to the Desktop, right click it, select "Open With..." and select "Programming - Python 2".