You mean the thin, no bass sound? Nasal twang?
Drop a graphic eq on the track for easiest/fastest work
Start rolling off all bass at 500Hz.
Boost the audio by 4-6dB at 1K.
Boost again, 4-6dB at 5K.
Roll off high freqs after 10K.
This will give you a very thin, AM radio sound, kinda like what you'd expect to hear back in the 60's.