37b5 MAME core is the issue for sound. Speed seems manageable on default, better at 233. 0.53 core adds sound for it. Expect it to slow down even more then . Maybe if you asked Franxis really nicely he'll consider individual tweaks..
Thanks for your thread. It's the first time i've seen this game, but im interested in giving it a try due to the association with Metal Slug. Irem made a lot of cool PC Engine games also.
This is a brilliant game and as you say shares lots with Metal Slug, including a few of the developers i think. The art style is practically the same too. Would love to get it working at a good speed with sound!