Simple VU meter with Lm3915
Russell mentioned probably more ways to solve the problem than you care about, but that's Russell: always gives extensive answers :-)  

The Greinacher multiplier he mentioned at the end of his answer may work, but the diodes may mean that you already need a rather high input voltage, I don't know if it will work well on the line output. (The Greinacher is typically used to multiply power supply voltages, not signals. It can be used to get kilovolts from a 110V or 230V line voltage, for instance.)  

Russell also mentioned this opamp amplifier:  

  

This is one of the basic opamp circuits, and is easy to understand if you know some basic things about opamps:  


The ideal opamp has infinite amplification  
The output signal is the amplified difference between the inputs
The ideal opamp has zero input current  


The first two items imply that you'll get a finite output signal if both inputs are almost at the same level. And that's what an opamp in a negative feedback system will do: it will try to make the level on the - input (called the inverting input) the same as on the + input (the non-inverting input). So that will be \$V_{IN}\$, and the current through R1 is  


  \$I_{R1} = \dfrac{V_{IN}}{R1}\$


Since there doesn't flow current in the - input the current through R1 is the same as the current through R2, so the voltage over R2 is


  \$V_{R2} = I_{R1} \times R2\$  


and therefore  


  \$V_{OUT} = V_{IN} + \dfrac{V_{IN}}{R1} \times R2\$  


or  


  \$V_{OUT} = (1 + \dfrac{R2}{R1}) \times V_{IN}\$  


So, if we choose R1 = 1k\$\Omega\$ and for R2 a 100k\$\Omega\$ potentiometer, we can vary the amplification between 1x and 101x. This will be a good amplification for a line out signal, and you can set the amplification to the LM3915's full scale.