Total novice question here so I apologize in advance. I’m helping my local church with their live stream.

Here is the equipment:
Brand new PC (meets vMix recommended hardware requirement)
HD SDI PT20XZCAM (PTZ Optics)
Inogeni SDI2USB3
Existing soundboard (analog only, no USB)
Behringer UMC-22
Three monitors (one for desktop running vMix program, one for desktop capture of Easy Worship, one for final mix) also using taking the final mix output and distributing it to several TVs throughout the church via HDMI)

Here is the setup:
PT20XZCAM to usb capture (SDI2USB3) plugged into PC
Soundboard to line in on Behringer UMC-22, plugged into PC
3.5mm mini line out from PC plugged into soundboard (for in house audio from PC)

Here is my dilemma; the computer was upgraded because the old one could not support the demand of vMix. With the old PC I used a 3.5mm splitter to supply a line out to the mixer as well as a pair of headphones for monitoring what was going live. With the new PC (using the same splitter) I am getting a decent output to the board, but the volume in the headphones is lacking. Aside from amplifying the headphones is there anything else that I may be doing wrong? The strange thing is that I am splitting the output on from the PC and one is going to the soundboard (which is okay) and the other is going to the headphones (which lacks in volume). Also, is this the best way to connect a PC to the board and vice versa? Again, I am new to this and I am just trying to help them get set up. Thanks in advance for any help. If there is any more information needed, just let me know.