Broadcast standard.

We just drove through Iowa and spent an evening in Des Moines with friends.

It's true what they say. I didn't hear a single accent the whole time I was there.

Except for a young boy about a year old who was practicing his linguolabial stops. I think they were voiced because he wasn't aspirating them. They sounded a lot like [b(ə)].

A linguolabial is marked by a subscript 'birdie' (or 'seagull') diacritic. The coronal articulation is noted by the [d] and its labial placement gets the diacritic [   ̼]? The IPA doesn't recognize a [b̼] because the [b] would be a bilabial and the linguolabial doesn't use the lower lip. The 't' and 'd' are the most reasonable coronals to use because symbols like 's' 'n' 'θ' 'ð' 'ʃ' 'ʒ' etc are fricatives not stops. The other coronal stops 'ʈ' and 'ɖ' would make little sense because they are retroflex symbols. Why would a symbol used for a retroflex be used in a symbol for the opposite of retroflex?