Old radios had limited frequency ranges. Use an EQ plugin to cut out the deep bass and very high treble. A good starting point is a High Pass filter around 600Hz and a Low Pass filter around 3.5kHz .
Use the "Trickster" or similar voice-changing filters in the audio editing menu after recording a clip. Old Radio Sound Effects
Use the "Scientific Filter" and select a curve that emphasizes the mid-range frequencies. Old radios had limited frequency ranges
Apply gentle compression to even out the levels, mimicking the way old broadcast hardware processed audio. 3. Quick Software Shortcuts Old Radio Sound Effects