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Tuning sound on prewar sets is usually very easy. As TDRyan pointed out, tuning has to be more precise when using FM sound with a prewar set, since the set has to be tuned exactly on the slope of the bandpass to work right. With an AM sound source, the local oscillator can be plus or minus 200 kHz or so and still produce good sound, since the IF stages are relatively broad (most are in the 4-8 mHz range). Some sets had their local oscillator adjustments hidden, since they didn't need to be changed very often.
As for the advantage of FM over AM in television audio, I think this is overstated. The video becomes badly degraded at relatively high signal to interference ratios, well before AM sound does. All FM sound accomplishes is to allow undegraded audio while the video is trashed. As for the higher fidelity of FM, just look at the speakers and audio sections of TV sets - it was rare for manufacturers to take advantage of it.
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