Question PC to receiver audio too quiet

[SOBS]

I just got the Yamaha Stereo Receiver RX-485, as well as a pair of SONY's SS-CRB5 I tested the speakers beforehand and they sound great, as well as the receiver (I tested both separately, the speakers with a different receiver and the receiver with different speakers), but when I plug in everything into my desktop PC the sound is way too quiet, I have everything at max volume and it barely makes noise. I am using the AUX input, I also tried the phono input but just makes everything sound muffled (very loud though). I already changed multiple cables, the ones from the speakers, and the aux but nothing seems to work. They sound way quieter than the speakers I had before (Thonet & Vander Vertrag)

Help please