Unfortunately for your setup, the motherboard has only a optical digital port.
Good news is there are easy solutions that can be very price friendly. Be aware that the below items are what you need to get the job done. They are by no means considered high quality or audiophile units.
By adding either of these, you would shift the audio responsibilities from the integrated audio portion of the motherboard to the newly added interface. You will have the option of using one or both of the interfaces simultneously. Totally up to you since you know your situation better than anyone else.
This is an internal audio interface (a/k/a sound card). You would insert the card in to one of the two PCI expansion slots on your motherboard. Look for the RCA connector, furthest from the bracket screw hole. That's the connection you're looking to use which it also on the rear input panel of your Philips home theater system.
Here is an external USB audio interface (a/k/a sound card):
If you want to keep shopping, just do some searches for "SPDIF sound card" or "SPDIF audio interface." Just be sure it has a SPDIF RCA or SPDIF coax (coaxial) jack.
An example of a S/PDIF coaxial RCA would be this:
S/PDIF is an acronym for the connector technology you need, it stands for Sony/Philips Digital Interface. It carries a digital audio signal over a special RCA cable or an optical TOSLINK wire. While you can use any RCA cable, you should look for a "SPDIF RCA" or "SPDIF coaxial" cable. This special cable has a specific impedance necessary for a healthy digital connection. More info about S/PDIF here: https/en.wikipedia.org/wiki/S/PDIF