Find some people who have a little vocal technique, but not too much, so they don't sound better than that? What specifically attracts you around this performance?
OK. If you're working with volunteers, it's a long tough road, and one you'll never manage unless you can personally do the thing you're asking of them. But a couple of thoughts.
Overbreathing, and breathing in the wrong way can both be problems. Have them think of breathing below the ribs (which is nonsense, because your lower ribs WILL move), and no more than is comfortable.
I personally find 600mg of NAC (N-Acetyl Cysteine) before I leave for rehearsal/Mass does wonders for my lung capacity. OK, it's doping in the vocal olympics. I'm 67; I don't care.
Bad unisons are not so much from bad ears as bad support/air management, or from not knowing where a note is. It's pretty easy for chanters to "intuit" where the half-steps are, and usually correctly, but never as certainly as if they actually know.
Generally speaking, pitch problems are a result of not knowing the music well. There are exceptions, of course, but few and far between.
As to achieving a good choral sound? Start with unifying the vowels. Then make certain your cues for starts/ends are crystal clear. There's more, but that's a start.
The question in the title is a question of vocal technique and choral management. Forums like this are very useful for finding obscure information, book information, online documentation, and perhaps networking. But this is fundamental stuff if you are going to be leading a schola. Reminds me of how-to videos on youtube. ANYONE can be a youtube mechanic. Until you take apart your engine and can't find the video for how to put it back together again.
The hard answer no one will tell you is you have to become a good vocalist yourself first. Take a good vocal pedagogy course, choral conducting courses (standard as well as chironomy), and a few years of voice lessons with recitals and quality choral singing under a director with very high standards. The above video was decent, but even that decent performance didn't just happen. It was the result of years of hard work. I will pray for you and your schola.
You also have to chant A LOT. (Both yourself, and as a schola.) I’d say it’s rather difficult to conduct a one-off performance of nicely done chant unless the singers are at least semi-professional. More likely, it will take years of chanting together to get a truly honed and uniform sound.
The schola director looks a lot like Anthony Smitha, who lives in that area. I don't know if I still have a direct line to him, but maybe it would be possible to be in touch.
What's interesting is that Smitha has a phenomenal vocal technique (he has been, and may yet in fact be, a Cheese Lord), so I would surmise that the sound they are generating is either the result of his not having had a long time to work with them, or a deliberate preference of the Fathers.
I second the doping method. I found great success myself with taking ventolin before choir practice. Also, get your members to take up playing the oboe.
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