But one option when a musician is not available is to use backing tracks, ie recordings of the hymn/song, which just contain the instrumental part, not the vocals.
These can also be useful for learning new material.
Many of the major church music publishers sell backing tracks for some of their catalog - and some even have self-contained backing track packages like "No Organist? No Problem".
But as well, some websites provide backing tracks for free-to-use hymns. These include: