DescentBB

My synths are in form of VST plugins and I usually use them with various MIDI players like VanBasco Karaoke player or foobar, either via MidiYoke/Savihost, or directly (foobar Midi Player addon). But can I play midis in the game itself or during recording with my software synths?

I've always been able to have good midi music (much better than DosBOX) in Rebirth out of the box with no problem. What errors do you get?

Outside of Descent (if exporting the midi) I believe I just use Windows Media Player.

No errors, but Windows default synth (that is used by Rebirth, I suppose) sucks compared to, say, Yamaha S-YXG2006LE. In fact it is probably the worst synth in the universe. Media plyer uses the default synth, of course. Just install a good software synthesizer for midis and you will never want to use the default one.

Any recommendation of a good synthesizer (perhaps whete can I get yours too)? I did install a soundfont for DosBOX, but it made no difference in Rebirth or Media Player.

Download foobar and MIDI player plugin for it
http://www.foobar2000.org/components/view/foo_midi
read the instructiions carefully. Then search for Yamaha VSTi plugins (plugin is a DLL). The best synth for me - both for classical music scores and for karaoke songs - happened to be the one I listed above, other Yamaha synths can be good too, google them, I can share if you don't find one. Most of them are on Russian sites, however. How to use it in Rebirth, I have no idea, but at least you will be able to play standalone MIDIs with it. Unfortunately Microsoft broke the MIDI playback in Windows 7, by disallowing to switch to non-default MIDI synth, curse them!

I've been using Coolsoft's VirtualMIDISynth, which is a third-party midi driver supporting soundfonts; better effects; and probably much else things, for a few years now and aside from the occasional easily-solved hiccup it's worked perfectly for me.
Being a driver it works with games, midi players or even Windows Media Player. About the only thing I can't get it to do is interface with FL studio but since that's got its own soundfont player the only reason to want to do that is academic. It doesn't replace the windows midi synth so as not to break anything but just about any program using midi should see it as the default.

I've also found a player from yamaha called MidRadioPlayer which emulates the XG soundset you might find on high-end hardware or, more likely, studio keyboards; and it sounds pretty good in itself, especially for drums. Only caveat is I've never been able to find a version in any language other than Japanese, so you may need to play around with options for a bit like I did to figure out what they are. Or you might be able to read Japanese.

Big thanks Alter-Fox! VirtualMIDISynth works just nice, allows to redirect MIDI output to MIDIYoke and then to the VST plugin host. Unfortunately it has no direct support for VST plugins, but at least it is not in Japanese... I am now thinking about re-recording some of my newer Descent vids with better music (namely Obsidian, the music is really awesome and I don't want to ruin it by default Microsoft synth).