Cantamus.app has 30 days trial then €5/month. Only one voice per stave, so it's not like a large choir, but it's a rough approximation. Cantai.app promises to be better but is fraught with teething troubles unless you know how to use a DAW. If you let me have a musicxml export of a score I can create a .mp3 from Cantamus for illustration purposes. My previous post is an example.
Computer recordings will not simulate chant in the best way. That is the reason I suggest (I think I need to indicate this in the score soon) that the chant like rhythm be observed. However you sing the chant should be the way all the other parts be sung.
Indeed, chant is the least successful use of machine generated singing, because unless the writer spends an inordinate amount of time tweaking the duration of each note it's just going to sound mechanical and not like following the text rhythm. Cantamus, Cantai, and the like are OK to create demos of conventional rhythmic modern notation eg Arvo Pärt Magnificat, such as for choir practice recordings or illustrative proposals for commercial work.
To participate in the discussions on Catholic church music, sign in or register as a forum member, The forum is a project of the Church Music Association of America.