I was wondering how I can say "from the sound of his voice I can tell that he is not feeling well".
Can I say "he does not sound well" in this case?
I was wondering how I can say "from the sound of his voice I can tell that he is not feeling well".
Can I say "he does not sound well" in this case?
Yes, this is fine. (Stating it in the affirmative would also work: “He sounds unwell.”)
However, it may be less specific than you want. It means that you have heard something, not necessarily the sound of his voice, that tells you he is unwell. (It could equally be the sound of his coughing, or someone else’s description of him, that you have heard.)
You could clarify your meaning in exactly the way you did in your question:
From the sound of his voice, he does not sound well.
From his voice, he does not sound well. [Shorter and doesn’t repeat “sound”.]
As opposed to:
From the coughing and sniffing, he does not sound well.
From what you describe, he does not sound well.