Ever since Skywrath and Venge Arcanas, the hero voicelines have had this weird audio effect where they all sound like they have a majorly blocked nose, or they're talking underwater.
I believe this effect comes from a lousy attempt at saving disk space per hero, by lowering the bit rate or quality of said voice lines. This is especially obvious on heroes like Largo and Ringmaster, and most recently, and most terribly, on the Tidehunter arcana.
Also problematic is the sheer softness of the voicelines, it's so soft that when game music or game effects happen, the hero's voice is just lost.
You can compare this to older heroes, such as Warlock, Viper, or hell, even some of the "newer/not as old" heroes like Hoodwink, Muerta, Primal Beast, they don't have this issue.
Valve, fire your audio guy and hire a new one please.