Matt, your questions are acknowledged. There is no need to double post, especially in the same thread.
First, I searched for the Uniden UH510-2 online and discovered it is manufactured specifically for Australia CB and New Zealand PRS frequencies. This is an international forum community of sorts, and we’re based in the US, so CB radio is different here, as it is in various parts of the world. As such, we don’t use the UH510-2 in this country.
According to the manual, which, in case you don’t have one, is easily available for download from Uniden Australia’s web site, the UH510-2 is an 80 channel handheld radio and operates on UHF frequencies between 476-477 MHz. I know this because I downloaded the manual for your radio myself and looked up the Australian CB frequency table online.
The BF-9700 operates on UHF frequencies from 400-520 MHz, so technically, they should be able to talk to each other. However, unlike the Uniden, in which the frequencies are hard programmed at the factory to their assigned channels, the BF-9700 needs to be programmed, which means the channels are not assigned to frequencies. You need to program the BF-9700 and assign a frequency to each channel.
In other words, the BF-9700 doesn’t just work out of the box, you need to program it first.
To answer your question about earpieces, nearly every handheld two way radio has a built-in speaker, which is the default listening mode. If it also has an audio port for an earpiece, you can use it as an option. When you plug the earpiece into the radio, it disengages the built-in speaker for private listening. If you unplug the earpiece from the unit, the radio re-engages with the front speaker and you can hear the audio from there. That’s pretty much all there is to it.
The short answer is, if you want to hear audio from the front speaker in the radio, unplug the earpiece, or simply don’t plug one in.