As of now, it's unclear which parts of Genshin Impact version 5.1 have the incomplete voiceovers. The lack of English voices in the above video leads many to suspect it will be the limited-time events, especially the Sumeru-centric one.
The SAG-AFTRA Strike:
The Screen Actors Guild-American Federation of Television and Radio Artists (SAG-AFTRA) is on strike against Formosa Interactive, a company associated with HoYoverse's voice-over work. The strike began on July 26 this year and aims for transparency regarding the use of artificial intelligence (AI) in the industry and other demands.
HoYoverse and Voice Actors:
This is just one of the voice actor issues HoYoverse has faced recently. The voice actors for Honkai Star Rail's Moze, Argenti, Huohuo, and Tail were suddenly changed. However, this was completely unrelated to the SAG-AFTRA strike.