Some scans of the Tri. theatre booklet are on the internet. These two feature Yamato's design and his harmonica prints (they appear identical to the Adventure ones, for obvious reasons). There may be more info in these but it hasn't been translated yet. I ran it through a translator and got the following:
For the harmonica section it reads (more or less): Harmonica: Blues harp fans know from when Yamato was young. Gabumon loves the tone. Yamato plays (well) when he is pensive.