First, does your monitor have an audio output? Some do, and it will output the audio from the HDMI/DisplayPort input.
If not, you could get an HDMI audio extractor (like this one) to split the audio out of the HDMI signal. Then you can get any streaming device that has an HDMI out.
If your speakers are not powered and you are not married to your amplifier, you could buy an A/V receiver.