It is recommended that when you wire up the headphone jack, you do not wire it up as a stereo jack. Inserting a mono plug into a jack wired for stereo (i.e. when connections are made to both the tip and to the ring connectors) will likely result in a short from the ring to shield when the plug is inserted. Several constructors have blown up their TDA2822M audio output IC as a result of exactly this scenario.
If you do want to wire the supplied headphone jack as a stereo jack, add a series 8 ohm 1/2 watt resistor (or perhaps a 4 ohm resistor) between the uBitx board and your phone jack. This should safely prevent any damage, though it will reduce the maximum available audio slightly through the headphone jack.
Wiring up the audio connector so that you can also use an internal speaker
The headphone jack can be wired to connect normally to an internal speaker, until a plug is inserted in the jack, whereupon the audio is routed to the headphones. Again, it is recommended that the jack be wired up for a single channel (using the tip only) to avoid problems when inserting a mono plug into the stereo connector.
The jack’s tip has two connections (one is switched off when you insert a plug). Because the sidewall on the jack is “see through” you should be able to see how this works in practice when you insert a 3.5mm plug into the jack. Wire the terminal that is switched off when a plug is inserted into the headphone jack, to the positive terminal of your speaker. If your speaker’s polarity is not marked, it won’t matter which terminal on the speaker is connected to the jack.
The negative terminal of your speaker should be soldered to the shield connection on the headphone jack. To be clear, the wire from the main board and the wire from the negative terminal of your speaker are both soldered onto the rear (shield) tab of the stereo jack.