Call init() before using beatsPerMinute.

libs/microbit/music.ts

@@ -162,6 +162,7 @@ namespace music {
     //% help=music/change-tempo weight=39
     //% blockId=device_change_tempo block="change tempo by (bpm)|%value" blockGap=8
     export function changeTempoBy(bpm: number): void {
+	init();
         setTempo(beatsPerMinute + bpm);
     }