MP4 is a container. It has video and audio codecs within that container. Roku devices support the MP4 container, but they have limited codec support. For video, the supported codecs are H.264, MPEG-2, and for 4K devices H.265. If your ripped files are using a different video codec, you get exactly what you are getting, audio but no video. You can use a program called MediaInfo to see what codecs are within your container.
The best program for ripping DVDs and Blu Ray discs is MakeMKV. It uses the MKV container, which Roku also supports. DVDs usually contain MPEG-2 video, so it's puzzling why your rip isn't working. But I need to know what codec is actually within your container to say where the issue is.
Without changing your current file, how can you play it? Emby is one solution, as is Plex. They will both transcode any unsupported codec or container so the Roku can play it. Both of those have dedicated apps for Roku devices. You can also use a DLNA server and use Roku Media Player. Serviio is the DLNA server I recommend (and I'm the author of the Roku profiles included in Serviio), but just be aware that Roku Media Player has some bugs that Roku hasn't addressed in several years.