If you run Dolphin in Windowed mode, you should see a little % at the bottom telling you overall game speed. If it's below 100%, that means you're not playing the game at fullspeed, even if you're not detecting visual lag (it'll be more evident though the greater the % decreases).
Audio is probably as good as it's going to get. Audio time-stretching was recently added to the Dolphin development builds, so using a recent build is the best you can do audio-wise. I haven't tried this myself, but I think I remember hearing that with the new audio time-stretching features, it works pretty well if you specifically limit your FPS in Dolphin to something low (e.g. 25 in your case). That might not be very accurate, I only glanced at those discussions.
Recent dev builds are here btw: https://dolphin-emu.org/download
Audio is probably as good as it's going to get. Audio time-stretching was recently added to the Dolphin development builds, so using a recent build is the best you can do audio-wise. I haven't tried this myself, but I think I remember hearing that with the new audio time-stretching features, it works pretty well if you specifically limit your FPS in Dolphin to something low (e.g. 25 in your case). That might not be very accurate, I only glanced at those discussions.
Recent dev builds are here btw: https://dolphin-emu.org/download