learning how to substitute any sounds for its intended instrument

[Vladv1]

"I now felt much more comfortable as a collaborator when it came to writing. Rock had his thing going, so Mickey Lee and I became songwriting partners. We wrote quite a few pieces on a Wurlitzer electric piano, in an out-of-the-way motel room that Mickey Lee was living in at the time. The instrument’s warm, milder sound made it harder to write big block-chord guitar-based tunes, but it helped me immeasurably in learning how to substitute any sounds for its intended instrument in my head, especially guitar, a handy skill to have in the coming years when I would work with some of the best guitarists in the world".
"Rainbow In The Dark", Ronnie James Dio.
Could you please explain the bolded in other words? What does "its"refer to? Thanks.

 

[Tarheel]

"I now felt much more comfortable as a collaborator when it came to writing. Rock had his thing going, so Mickey Lee and I became songwriting partners. We wrote quite a few pieces on a Wurlitzer electric piano, in an out-of-the-way motel room that Mickey Lee was living in at the time. The instrument’s warm, milder sound made it harder to write big block-chord guitar-based tunes, but it helped me immeasurably in learning how to substitute any sounds for its intended instrument in my head, especially guitar, a handy skill to have in the coming years when I would work with some of the best guitarists in the world".
"Rainbow In The Dark", Ronnie James Dio.
Could you please explain the bolded in other words? What does "its"refer to? [.........?????.............?????????.....]

He was able to imagine the sounds of the various instruments when he was composing a song.