Yeah the slot thing is hit or miss.
Monitoring externally to avoid the latency issues can be done a variety of ways, here are a few:
-Tap your preamp or mixer directly back to the headphone for the voice or instrument you are tracking, blend this with the mix from the computer.
-Same thing but for some reason people like this, route the computer mix to the left head phone jack and take a feed directly off your preamp or mixer and route the voice or recording directly to the right side.
-Forget monitoring your voice at all, just put up the mix and sing along, same with instruments.
-All this said, some singers, me included, want to hear their voice coming back in reverb. The solution, route your voice directly into a nice external unit after the preamp/mixer but take the dry signal to track and not
the reverb one. This allows the vocalist to work with some ambience and hear the mix.
I like this last one the most. Later I can route back out to the external reverb if I like its vibe or use an internal plug in.
Hope that helps. It's all in routing prior to the computer while monitoring the music.
Yikes, forgot the most important thing, don't route the track you are recording to any output. Keep it out of the mix you are monitoring. If you don't have a limiter going into the computer, watch your levels or keep an eye out for distortion. If you have a seperate monitoring room, than this is not an issue.