Yoma 19b - ודברת בם (you shall speak in them)

This gemara offered an interesting slant on a question I had been discussing with a few people: We know you are supposed to say Shmoneh Esrai quietly (reaches your ears, not anyone else's). I was wondering if that means you're supposed to whisper (is that לחש), or just say it very quietly? That is, do you voice the words only quietly, or do you whisper where the voicebox doesn't hum?
Almost everyone I know whispers: spot checks in shul make this obvious. On the other hand, whispering isn't actually any quieter than mumbling quietly; whispers' high registers carry well. As a matter of fact, something is lost by whispering - Hebrew consonants that should be distinguished:
ס\ז כ\ג פ\ב ת\ד are some examples (give them all dagesh), can no longer be distinguished.

So here's gemara and Rashi
"You shall speak in them (Torah) - and not prayer. Rashi: Prayer should be בלחש, as it says [about Chana] 'Her voice was not heard.' "

So if לחש means "whisper", this would have an interesting reading: You shall not speak in prayer - whispering is not proper speech, it combines different consonants. Her voice was not heard - no hum from her voicebox.

If לחש just means "lower volume", the gemara is much less interesting, I think: Everything depends on volume.

Has anyone seen discussion on this question?

On reflection, I guess the other reading of the gemara is also interesting. It's not just a matter of volume. There's a speaking that only I can hear (just to my ears and no further), and a speaking that others can hear. The second kind is "really" speaking, proper דיבור, as it communicates with others. And that kind is only allowed for Shema, not for Tefillah.

The gemara that Rashi brought in Brachos (31a) seems to support the second way of reading (where volume is the only criterion). It learns from that verse from Chana that "אסור להגביה קולו בתפילה" - you are not allowed to raise your voice.