1) If original recording is Stereo, then convert to Mono.
2) Lower the volume.
3) Use the EQ system to reduce the highest and lowest frequencies. This will make the voice less distinctive. Alternatively, one could try raising the EQ on the middle frequencies (guess: 300htz to 1khtz)
4) Add reverb if the voice is close to a wall, or it is a narrow street.
5) Add some background sounds (the kinds of things one might hear in an empty street.)