I've seen that scene several times... I only hear exhaling breath, and only his lips opening, without moving more, which seems like he's just opening his mouth a bit to exhale after sniffing the shirt so hard... you guys sure it's not just psychological? I mean a breath can easily be interpretted as anything said under the breath, and when you want to hear it bad enough, you mind just interprets it as "I love you"...
Anyway, what was so wonderfully tragic about this story (and possibly its strength) was the fact that neither Ennis nor Jack openly called their relationship as love. To have Ennis say "I love you" would take this away from the story... I really hope the Heath didn't do such a thing, as it would make the story less of a tragidy.