z87e-itx audio/video

Hi everyone, I recently bought new hardware for my htpc and thus I migrated my "old" z87e-itx (including:2x4gb ddr, i5 4570s, 840evo SSD) to a new smaller case (fractal 304). I retired its pci gpu (assuming the igpu would be enough) and I'm currently keeping its seasonic psu (but I'll probably replace it since its on/off switch doesn't work properly). After powering it on I noticed that all audio/video played way too fast (media clips, mp3s, flash, even animated icons). I tried all the usual, driver related, routine. I did a clean windows reinstall, and I even tried booting from Slax. Same result. Temporarily reinstating the previous pci-gpu yields no effect, neither does fidgeting with any bios options (including restoring uefi defaults or updating firmware).
Does anyone have a clue about what could be the source of the problem? It doesn't look to be software related.
thank you in advance for any reply

P.S.:
in its previous htpc state it was connected via hdmi(pci-e gpu) to an onkyo amplifier, now I'm using a dvi/monitor connection (either pci-e or onboard as stated above)