Do line level circuits need shielded enclosures?
Does a line level circuit like a volume pedal need a shielded enclosure? 


No, But if you put your cell phone right next to a plastic box with a high impedance signal loop, poorly designed on the high impedance input, you might be able to pickup the tower re-sync transmissions. dah-dit-da-dit...  This RF often comes from input diode carrier demodulation to baseband and can be suppressed with an RF shunt cap.

Most poor SNR situations come in from radiated noise induced into conducted noise on the signal cables and from poor HF ripple on floating DC supplies.

This current transfers into a voltage from unbalanced impedances for signal and return. 

A range of supply capacitors for DC ought to be included from 0.01uF to 10uF to suppress AC hum to RF induced noise currents.