Fix crash with more than one audio stream #47

Job	Run time
build-linux (gcc-9)	6m 41s
build-linux-Qt6 (gcc-9)	8m 40s
build-macos-brew-Qt6 (clang-9)	2m 52s
build-macos-brew (clang-9)	3m 11s
build-macos-no-brew (clang-9)	6m 31s
build-macos-no-brew-Qt6 (clang-9)	5m 30s
build-windows-mingw (5.15.*)	7m 18s
build-windows-mingw-Qt6 (6.3.0, qtmultimedia qt5compat)	10m 26s
build-windows-vs (5.15.*)	7m 16s
build-windows-vs-Qt6 (6.3.0, qtmultimedia qt5compat)	12m 54s
	1h 11m 19s

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