98 Altima Keyless Entry Problem

i just recently bought a 5 spd 98 altima for a commuter. when i bought it, it came with only a ket and no keyless entry remote. i know my car came with a keyless entry system because all altima SEs in that year came with it stock. the remote i ordered is still on its way.

problem: sometimes after i turn off the engine, take out the key, get out of the car and lock the car, the audio turns back on after a good 15-30 seconds and keeps playing until i get back in and put the key back in. does anyone know if this is a keyless entry security problem? or an audio/electrical problem?