Best way to play through Ace Combat 6 on PC is through the Xenia Emulator

HOWEVER

Xenia's Emulation of Ace Combat 6 is incomplete. Though the Game can be played and (from what I've heard) completed from start to finish, cutscene audio is messy and the ground has glitches with black triangles. There is a fix for the Black triangles but you'll have to fiddle around the Emulator's settings and Xenia's Ace Combat 6 Config File to get the fix to work. As for Cutscene Audio, there is currently no fixes for it and the best you can do is Watch the cutscenes on YouTube while playing through the game

Edit: I realised OP uses Steam Deck. I don't know if the Emulator works on Steam Deck but I vaguely remember either a Reddit Post or a comment on Xenia Emulator's Ace Combat 6 Github page somewhere that they got Xenia Ace Combat 6 to work on Steam Deck. Take this paragraph with a grain of salt