I need it to run Sonic Robo Blast 2 Final Demo. Every other dependency needed for it is available in 15.6 (such libSDL2-2_0-0-32bit and libmodplug1-32bit), except Mixer. I don't know why it's missing. Perhaps it's too old. This would explain why libmodplug1-32bit is missing from Tumbleweed. This would not, however, explain why libSDL2-2_0-0-32bit is still in Tumbleweed.