The round barrel connector has five pins for a common Honda type corded headset.
That is a SENA specific cable that is part of a system to use a BLUETOOTH headset instead of the original corded type headset so it will not do what you want.
There are similar cables that allow you to use a common plug headset instead of the Honda type headset. These are for audio only - no intercom or CB.
https://electricalconnection.com/index.php/product/din-to-3-5mm-headset-adapter/All the above are for headset use, nothing related to speaker audio.
If you want the audio from a headset type jack to be injected into the Honda factory system, you can use a Bluetooth or corded cassette adapter. You can also insert a unit between the antenna cable and the radio that will add a cable to plug into your music source. There are other units that plug into your music source and broadcast out to a FM frequency so just tune the radio. These choices will allow the audio to be heard using either speakers of headsets.
Frankly, the best answer is disregard the Honda system altogether. Get a new Bluetooth headset and pair it to the phone and be done with it. Bike's speakers are probably pretty tired anyways.