[Hybrid tellies]

This graph shows the response of an AF band limit filter used by most AM transmitters to limit interference to adjacent channels. As you can see it does not cut off at 4.5Khz but tails off at 6Khz.
AM receivers that have a similiar response curve the audio can sound reasonably pleasant even with music.
The problems occur when AM receivers, especially those built in the late 1970's and for much of the 1980's were fitted with brickwall or narrowband IF filters which cut off all audio above 4Khz. The AM audio quality on such receivers was horrendous especially with the heavy compression that was used at the transmitters back in those days.
Another point to bear in mind is that even if the broadcasters were able to provide a wider bandwidth most AM receivers would not be able to benefit from this.