Yes 380/min = ~6/s which is a very open ptime of >100ms, this can also be dynamic and change don the fly. It ultimately comes down to how big the packet can be before it gets split which is a function of MTU.

If you have 50ms of latency between parties, and you are sending 150ms segments, you'll have a perceived latency of ~200ms which is tolerable for voice conversations.

One other note is that this is ONLY for live voice communication like calling where two parties need to hear and respond within a resonable delay - for downloading of audio messages or audio on videos, including one-way livestreams for example, this ptime is irrelevant and you're not encapsulating with SRTP - that is just for voip-like live audio.

There is a reality in what OP posted which is that there is diminishing returns in actual gains as you get lower in the bitrate, but modern voice implementations in apps like whatsapp are using dynamic ptime and are very smart about adapting the voice stream to account for latency, packet loss and bandwidth.