This is what David Hayter, the English voice of Solid Snake and Big Boss, posted on his Twitter the day before yesterday:
Paul Eiding is, of course, the English voice of colonel Roy Campbell. So what could this be for? The most exciting theory would be something like MGS5, but it is more likely that this is about something less significant. The safest bet would probably be voice recording for the Vita version of the MGS HD Collection (some new lines for the specific buttons, as was recorded for the console versions as well).
It could also have something to do with the 25th Anniversary of the series, which is this year. Maybe there will be a short movie to celebrate, or something similar. It could also have nothing to do with Metal Gear at all, Kojima could be using the same voice actors for different characters in (for example) Project Ogre.
In any case, whatever this is about, we will probably learn soon enough. Or not.
Source: David Hayter’s Twitter