The standard says (RFC 4566 / 8866):
"For audio streams, encoding-params indicates the number of audio
channels. This parameter is OPTIONAL and may be omitted if the number
of channels is one, provided that no additional parameters
are needed."
This means that the number of channels is 1, if not specified within
rtpmap. It does not state that the codec's default value shall be used,
nor static payload definition.
Addionally the encoder always defines the number of channel. As the
encoding-params MAY be omitted, they are added to the rtpmap line, even
if the number of channels is 1.
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