https://archive.org
The Internet Archive digitizes and maintains archives of CDs, LPs, and 78rpm records. For the most part, these archived items would fit under the Release-URL relationship scheme. For example:
• https://musicbrainz.org/release/4599ec40-63b0-4529-88e3-af8be895c1ac = https://archive.org/details/cd_interstellar-space_john-coltrane
• https://musicbrainz.org/release/b57579db-6192-3978-a451-3f034b484ddd = https://archive.org/details/facemusicsoundre00elec

The bulk of 78rpm recordings in the Internet Archive are represented as per-side (i.e. a single side of a 78rpm disc) rather than per-release. These might be more appropriate as Recording-URL relationships. For example:
• https://musicbrainz.org/recording/f4279cd9-2e7d-4668-9cf9-859405220f68 = https://archive.org/details/78_un-refrain-courait-dans-la-rue_edith-piaf-chauvigny-luypaerts_gbia0011653a
• https://musicbrainz.org/recording/96050011-3713-4396-a343-0a427f55de5e = https://archive.org/details/78_la-vie-en-rose_edith-piaf-louiguy-luypaerts_gbia0011653b