I am very likely that one case, since I even posted a video comparing all M generations.
So let me be clear. The M4 speakers are NOT better. They are just slighlty louder.
You can only tell if you compare them side by side (that what I just did to check again).
Not many people have M4 and M5 13" to compare and nobody will tell you one or the other is better since there is no better, and the difference is small. Same with M1 vs M4, the difference is very small.
2018 12.9" vs M1 12.9: much bigger difference. M4 vs M5: minor difference in volume that's all. More in the "roughly the same" camp than anything very noticeable.
Have I compared several M4 and M5 with the same results to avoid having a "special device"? YES. I have had 3 M4 (sold 1) and 2 M5 and they were all the same. Last time I compared the old M4 with the old M5 and now I am comparing the new M4 with the new M5, with the same difference.