^^^What they said ^^^
All's I heard was a single acoustic guitar at the beginning recorded in stereo, not overly bright, but mic'ed fairly close. After that comes in what seems to be a mono guitar track panned pretty hard right. And then some voices with little to no
effects or ambiance, also mic'ed fairly close in a drier space than the guitars were recorded.
The compression on the guitars sounds very smooth and unobtrusive, transparent. So try something like an LA2A emulation or something soft-knee or optical with conservative settings. The eq wasn't very hyped or overly bright on the entire track, so when eq'ing individual sources, try to keep the curves wide, musical and
natural, unless you need narrow notches for cutting out problem frequencies you can't help, since you didn't record it.
Compression on the voices was maybe a tad faster than on the guitars because the voices sounded pretty up close the whole time, but it was still rather gently handled.
As far as
effects the only thing I heard was a nice room ambiance on the guitars, and maybe a medium plate-style
reverb for the tails. The vox sound like they are sent to that plate a a bit too but they are pretty damn dry. The only other effect I heard was a bit of creativity with the panning on the female supporting vox. At first you hear them slightly to the right, making it sound like she's sitting right next to the lead singer (and you can hear her guitar over there too). Then later it goes into the center when the rhythm excitement picks up a bit.
So that is what I am HEARING. It really didn't sound very "big" to me. Open? Sure, because there's not much there to fight. But everyone's take on describing a song's sound is different. Take whatever you will from the description of what "I" heard and use that to judge if you can achieve what I described they are doing, but with the material you have been handed. If the guitars sound small and thin, out of phase, etc, there's not much you can do to magically make them sound fuller or more upfront. Good news is, if that's all that was recorded, and if it was done poorly, then at least it won't be a huge deal for them to re-record the bad
elements upon your recommendation. If they don't want to, hand them your mix with the best of your abilities and they will see magic is just a fantasy.