I moved your thread to the "Sequencers" subforum although I'm not sure which subforum is the best fit for it.
Which computer operating system does your laptop use?
What connection jacks/ports does it have?
What program are you using to play the MIDI files on your computer and send the MIDI to your Motif rack?
I'm thinking you probably just need a good (or maybe a better) MIDI sequencing program to play the MIDI files and send the data to the Motif rack. If you're using a Windows laptop, my first thought is to suggest a program called SynthFont. It's sort of similar to a DAW in design, but is more of a MIDI sequencer and MIDI editor. There is a free SynthFont 1 version, as well as a paid SynthFont 2 version.
If you go that route, you would need to load a given MIDI file, set it up in SynthFont the way you want it, and save the setup as a SynthFont file so you don't have to keep going through the same setup process each time you want to play the file.
As far as setting it up the way you want, what I mean is that you would select the desired output for each of the MIDI file's tracks so that SynthFont will send each track's MIDI data to the Motif to be sounded. You would also need to edit each track's settings to select which Bank and Program-- that is, which voice-- you want the Motif to use for sounding that particular track or MIDI channel. You can also use the MIDI editor functions to add, change, or delete any system and channel messages for things like Reverb, Chorus, channel volume, or even make modifications to each voice's envelope and filter settings (attack, release, cutoff, resonance, etc.).
Once you've got everything set up the way you want it for that particular song, save it in SynthFont so you can easily load it and play it back whenever you want.
Most DAWs can be used to do the same sort of thing, although DAWs can vary a great deal one from another as far as how much they let you get into the nitty gritty of MIDI messages, especially as far as things such as SysEx messages for selecting which Reverb, Chorus, and other Effects types you want your MIDI sound modules to use. Also, since DAWs are really intended more for recording and editing both audio and MIDI, they tend to have a lot of additional features and functions that you won't need for what it sounds like you want to do.
As far as sending everything to external speakers, I would suggest that you just hook the speakers up to the Motif rack so the Motif can send its audio output directly to the speakers. That is assuming you aren't wanting to have any of the MIDI tracks be sounded by other sound modules, such as by any virtual instruments, soft synths, or soundfonts running on your laptop.
If the latter should be the case, then you might want to buy a small mixer so you can connect the laptop and the Motif to the mixer's audio inputs, then connect the mixer's audio outputs to the speakers.
Or, if you prefer to do everything through your computer, you'll need to connect the Motif's audio outs to the laptop's line in (if it has one), then connect the laptop's audio out (which might be labelled as a headphone jack) to the speakers.
If your laptop doesn't have line in or aux in jacks-- which would be stereo, as opposed to a mono mic in jack-- then you could connect the Motif's audio outs to a USB audio interface and send it to your laptop via USB.
The connections are really going to depend on which model of laptop you have, and what types of connections it has available, as well as whether you're using any additional equipment such as a mixer or audio interface.
And your options for a MIDI sequencer will depend on which computer operating system your laptop uses.
