You gotta look in between.
Between Infinite and SSLP, there is D12's Underground EP, which has a song called Filthy. In it, his voice sounds like it's transitioning from Infinite to SSEP. Same with SSEP to SSLP. There is an increase in nasal tone and clarity. Listen Murder Murder and Rock Bottom. Now listen to If I Get Locked Up Tonight. It's heavily SSLP voice but it's starts the slight deeper tone change to the MMLP voice. Yes there is a change. Listen to I'm Back and Rock Bottom. I'm Back is slightly deeper.
Now move on to D12 Album Devil's Night. Compare I'm Back and the song Revelation or the song Fight Music, both from Devil's night. You will notice more layering and a slight tone change. This is his change into TES voice. Compare I'm Back and Fight Music and you'll see.
Now you go on to TES. Listen to Sing For The Moment. It's very similar to Fight Music. Nothing more to say other than it being different than I'm Back.
Move to Hail Mary 2003 remix, the Ja Rule Diss. This shows his voice even deeper than in TES. He stops layer and starts to use a "normal" voice. It's definitely deeper than before. Then we move to Encore. Best showcase of voice change is the song Puke. During the intro of the song there's a part where he says "there I go thinkin of you again". This part is nearly 100% identical to his 2010 speaking voice.
Next we go to When I'm Gone. He's using layers again but his voice is deeper yet not as deep as it is now. You'll notice a difference between Puke and When I'm Gone.
Now we go to The Re-Up. Wherever you really hear him, his voice is a deeper Encore voice. Sometimes layered, sometimes not. I can't really give any examples because I only listened to that album once. One exeption is No Apologies. It's clearly recorded in around 2002.
Now we go into unknown territory. This is when his biggest change occurs. This is when he's the most fucked up with drugs. In this era, some songs are The Apple, G.O.A.T., 50 Ways, and Wee Wee. All these songs sound like a deeper Encore voice layered OR a slightly higher pitched Recovery voice layered.
Relapse is a bit weird. The voice is like an extremely slightly higher pitched Recovery voice layered, but also a lot like a layered and deepened and accented Encore voice. But this is only in some songs like Medicine Ball or Drop The Bomb On. Songs like Beautiful are reminiscent of the drugged up songs I mentioned in the previous paragraph.
Now we move to Recovery where his voice is just not layered or overly layered and aggressive Relapse voice. If you listen to On Fire though, you'll notice it's just his speaking voice that was identical to the intro of Puke.
Next is Hell The Sequel which is some extra layering of Recovery voice and no real difference. Ex. Reunion and Fast Lane.
Now it's onto My Life. It's a heavily layered Recovery voice that's actually on a slightly higher pitch.
Next it's MMLP2. Here the voice is basically completely unlayered. He's also forcing himself to be more nasally. It's definitely noticable on songs like Rhyme Or Reason.
This trend continues to Shady XV. And sometimes it's even more high pitched. On the song titled Shady XV, he goes full SSLP voice for a second and flows his SSLP flow too. It's very very spot on to his old stuff.
And now finally he's back at a Recovery + TES mixed voice. This is extremely noticable on Bad Meets Evil's All I Think About song.
FUCK. I hated typing all that. This so embarrassing cuz it's shows how much of a Stan I am really. I don't care that much actually. I'm very critical of his music ever since Shady XV dropped. I hated it. Anyway, the voice thing has bothered me too. It's not so insane if you notice it change, but if you compare 2015 to 1999 it's bonkers.