The free utility TSRemuxer can be used to remux the .m2TS files of the BD and by simply ticking a box you can get it to output just the DTS core or AC3 core of the HD audio format streams. No reencoding required so pretty fast.
Or if you have the paid for version of MakeMKV, that utility can output just the non-HD audio data while it is ripping to create the BD ISO file but you have to pay for the tool to enable the BD support of the program (it's free for DVD ISO ripping).
I'm not sure whether TSRemuxer can directly accept BD .iso files as input, but just extract the .m2TS files from the ISO's stream folder (the biggest file is usally the movie) and run it though tsremuxer and the resulting file will play fine on the sumvision. Since the sumvision can't do BD iso menus there isn't much point in keeping the BD ISO so I just rip out the .m2TS files and keep those instead.