Is the 555 necessary here, or can I just use an op-amp?
Here's a link to another answer that should prove useful. Here's the diagram: -



Q1 and Q2 transistors "detect" audio and force a DC level onto C2. C2 holds that voltage and slowly (relative word) discharges via R2. Q3 is just a buffer to drive an LED.

Also, take note that a DC powered solenoid is usually designed to have significant resistance so that it can be powered by a DC voltage without infinite current flowing.