That depends on what country you are in.

In most of the world, all his works will be copyright until the end of 1966.

If Pange Lingua was published in 1916, was the copyright renewed? The library of Congress has the renewals. @CHGiffen last time someone asked about copyright I think you gave an informed answer.

From :- https://www.gov.uk/copyright my emphases

You get copyright protection automatically - you do not have to apply or pay a fee. There is not a register of copyright works in the UK.

You automatically get copyright protection when you create:
original literary, dramatic, musical and artistic work, including illustration and photography
original non-literary written work, such as software, web content and databases
sound and music recordings
film and television recordings
broadcasts
the layout of published editions of written, dramatic and musical works

The length of copyright depends on the type of work.
Type of work How long copyright usually lasts
Written, dramatic, musical and artistic work 70 years after the author’s death

[EDIT]This law - The Copyright, Designs and Patents Act 1988, was retrospective.

Copyright is implicit in the US now for works created here, but the rules that apply to older works depend on when they were created and where they were published.

The impression I'm getting now, though I haven't verified this, is that US copyright law respects foreign law in a lot of cases, but only for works after 1923 or so, which would put MacDougall's work in the public domain.

Public domain now applies to works published before 1930. And, "Public Domain Day" (January 1 of each year) is a gift that keeps giving; next year it will be 1931. Ron Krisman is correct, it only applies to the work as it was originally published, not to any later editions or versions. It also applies to books, films, and printed scores, but not to sound recordings, which are governed by different laws

Here is the address to the Music Library Association's Copyright page. It's geared to librarians, but the information there may be helpful to church musicians:

https://copyright.wp.musiclibraryassoc.org/

I often wonder how many copyrighted alterations of public domain texts meet the "Threshold of Originality" to genuinely qualify for copyright. Some things I've seen online, for example, suggest that simply updating archaic text, changing a few words, correcting typos, etc... wouldn't be enough. There appears to be no exact formula to figure out if the alteration introduces "at least some minimal degree of creativity." I guess that barring legislative or judicial guidance, each adaptation in question would have to be litigated, but I wouldn't be surprised to find more examples of copyfraud than genuine copyright.

Somewhat related, I've seen it suggested that there's no way the current Abbey Psalms and Canticles could possibly be copyrighted as there are so few changes from earlier public domain texts when it's all lined up next to each other. A PDF was prepared with a scan of the APC, preceded by the scanner's argument that they were not copyright... I can't remember exactly who shared that at the moment.

What I was told that thirteen or more textual alterations of a P.D. text qualify to make it an original work. I cannot, however, verify that. That was when I was working on the hymnal that came to naught.