No Roku device has speaker level outputs. The Express+ has analog audio outputs, but you would have to use an amplifier to power the speakers.
The other issue is the video connection. Not a lot of computer monitors have HDMI inputs, which is the only HD output that any Roku now offers. The Express+ has an analog video output, but that is not HD.
My suggestion would be to get an inexpensive HDTV with an HDMI input. That way you could use any Roku player, get HD video and the TV would play the audio with its built-in speakers. BestBuy has an Insignia 22" 1080 TV for $70.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.