You could use direct record but it's still not the best method, I'd still recommend you to just export your track and rap over the top of it in a different DAW - I'll still explain the steps with 1.7, but as you'll see it's needlessly complicated for your purpose.. the mpc excells at making beats, but tracking vocals/rap over the top is always going to be easier and better in a full fledged DAW
Direct record lets you record into a single sequence on mpc software. For rapping across a whole beat thats gonna be awkward - for a start you would probably have to convert your beat from individual sequences to a single sequence using song mode.
For the actual rapping recording hit Sample Rec and before you start recording set the input up correctly (you've got yours set to R and L even though you're only running through one line, highlight that field on the lcd and increment it with the jog wheel/plus/minus buttons or change it with your mouse in the software to make it set either to only R or only L depending on which input you're using)
Make sure your beat (all in one sequence) is playing before you hit Sample Rec for direct record to be in effect, when you're ready to come in with vocals on the beat hit record and go for it.. Now where you're going to run into problems is if you make a mistake and need to redo just one line, or one word, that's going to be extremely tricky to do using mpc programs and events.. you would have to completely re-do stuff, whereas in a DAW it's easy to delete an isolated portion of what you've recorded and overdub the correction quickly... you're also going to have trouble navigating through the sequence, essentially relying on numbers with no indication of the waveform you've recorded.. in a DAW you can see the waveform of your recording, so you can see at a glance where the verses/chorus is.
Direct record isn't an 'Audio Track' that we had in mind, when people talk about audio track they're usualy referring to DAW style audio track, where audio is represented as clips with waveforms that can be looped, stretched, cut, moved around within the lane, regardless of any grid/bpm etc.. I don't really see where audio tracks would fit in with the MPC software personally, because they're only really useful for when you're handling big recordings like a full 3 minute track of rapping, and where would you even slot that into the mpc software? Song Mode? I don't know
