Abraham heard God speak in the vision. He didn't hear the vision.
You're making distinctions that simply don't hold up under the general classification of consciousness as loudness. Suppose for instance I dream of eating pizza. Did I see this dream? Or did I taste it? Those kinds of distinctions break down. And if, during the dream, I saw and heard people speak, did I see the dream? Or did I hear it? Both, because those kinds of absolute distinctions/dichotomies just don't hold up.
Abraham beleived what he heard. Same as the Galatians did. That is the point of the comparison in Galatians 3. There is nothing in the text about the Galatians seeing revelatory visions or seeing anything else.
Abraham heard/received the divine Word of Direct Revelation (not a Bible), and thus received the Spirit through the hearing of faith.
Loudness is what makes my reading of Galatians exegetically strong. The Inward Witness MUST be defined as both vision/voice, for reasons stated.
"Did you receive the Spirit by observing the law, or by the hearing of faith?...After beginning with the Spirit, are you now trying to mature by the flesh?"
They heard the Voice in tandem with receiving the divine Word. They received the Spirit by the hearing of faith. At verse 6, he turns back to Abraham's similar experience of hearing/receiving the divine Word. Abraham heard the same Voice!
Your claiming that's not the parallel. This claim might carry some weight if the Inward Witness could be defined as something OTHER than voice. But the principle of loudness implies that both Abraham and the Galatians heard God speak. It's a perfect parallel.
After all, the Galatians received the Spirit. If the Spirit has no influence upon our mind, He is USELESS. And the moment He influenced the mind of the Galatians - that's the Voice! Since consciousness is loudness, ANY impact of the Spirit upon the human mind counts as the divine Voice.
To summarize this post, you're so desperate to disprove my solid exegesis that you are forced to resort to bogus, arbitrary distinctions that, under close scrutiny, simply don't hold up.