The obsessed may be get blinded, dumb, deaf or mad under the effect of the obsessing Spirit (the Spirit who caused obsession).

But the Spirit causing obsession is itself blind, dumb or deaf!, This seems somehow strange.

How did the deaf Spirit hear Jesus and respond to Jesus's order?

How to solve this matter in Mark 9:17, 25?

The text is clear in Greek text, KJV and ASV: the Spirit is dumb and deaf.

The text in ESV: Mark 9:17 (the Spirit made the obsessed mute), while Mark 9:25 (the Spirit itself is mute and deaf).

The text implicitly, but not expressly, indicates that the boy was dumb and deaf.