Changing to 50 didn't change much.
Well, I copied the sounds.bsa over to another folder and extracted it and in the sound\fx\wpn\weaponhit\bladeflesh directory there are three files named wpn_hit_bladeflesh_01 - 03, but in the CS I only found 2 that applied to this test. My character is completely unarmored and the NPC hitting me is using an unenchanted blade, so I assume the only sounds that should be playing when I get hit are the two currently in the script. The rest do not have flesh in the name nor sound anything like what I hear when I am being hit.
As for mods, deadly reflex is the only one that adds sounds during combat, but those are only the killing blows.
Ok, out of curiosity I decided to bring up my stopwatch on my phone and hit lap each time the messagebox popped up to see if there was any sort of pattern, and the results were kind of interesting. Well, I found it interesting anyway...no idea if it's actually relevant but here is what I got.
1 Hawkhaven guard hitting me with his silver longsword, each time the message box came up I closed it and hit lap at the same time.
0:09:50
0:19:28
0:24:24
0:34:37
0:44:35
0:49:31
0:54:27
1:04:19
1:14:18
1:24:17
1:29:20
2:09:55 - the guard decided it was a more logical idea to swing his sword 5 feet away from me during that time, but that still didn't break the chain
2:24:15
2:34:21
2:39:17
2:54:26
2:59:29
Like I said, no idea if this is relevant, but I found it interesting nonetheless.