@zozo, I have a theory about that Marcel...I think it's probably something to do with certain frequencies stimulating growth.
Outdoors those frequencies are perhaps provided by natural environmental factors. Indoors those factors are largely absent and music might provide a substitute...perhaps
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For instance, I read somewhere that Marram grass growth is stimulated by shifting sands and in particular in to developing a deep root system.
Another analogy would be turf growth in stadiums. In totally enclosed stadiums it doesn't grow as well as it does in stadiums without a roof despite climate control, fertz and all the rest.