How did he know it would produce a noise like a clap of thunder if it had never happened before ?
Perhaps he had heard the "snap" of a high velocity bullet flying overhead and correctly surmised that an airplane might make a similar, if higher amplitude "snap". More probably, personal recollections of witnesses are subject to "revision" by subsequent events.
In any case, I don't doubt that "Wheaties" Welch could have gone supersonic as described. However, it will remain "unofficial" since no telemetry or flight recording was dedicated to this flight as it was with Yeager's X-1 work. Nevertheless, Welch is a legend!
Best regards,
Westhawk