It depends on how old your computer, TV, phone, radio, etc. is. The newer ones pretty much all only allow one output, so if you plug this into the jack then the speakers will go dead. The older ones may also do this, and you may need to get wires to fit the TV's audio output that also merge into this headset's input. But on occasion an older one may allow both.
If it so happens that you cannot listen to both at the same time, then the cure for this is to look on the internet to order a "signal splitter or switcher" that has as it's "signal input" a hole that fit's this unit's "output" hole, and that also has then going out of it, "downstream" as it were, two or more output jacks: a hole or "jack" that fits this headphone's input hole and then one other lead to match another, external speaker (which means you also need an external speaker to plug it into). It's a little messier but we do it with our computer and also our Samsung TV (messy with the TV because the TV output is an "optical audio" cable and it is a mess getting it transformed into a traditional audio signal) because you can only get a single "optical audio" signal coming out of the TV for sound. Messy, but it works. She doesn't hear well and I do, so she does the headphones and I use the regular speakers.