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#21
The manual is a bit vague so hard to give a definite answer but your receiver does have an optical in. However it does not say what formats it supports. An external USB sound card such as the Sound Blaster X-Fi Surround Pro 5.1 (see link) has optical out. This should work. You will also need an optical cable (assuming you do not have one). They are also called a TOSLINK cable.
Sound Blaster X-Fi Surround 5.1 Pro - Sound Blaster - Creative Labs (United States)
It’s unfortunate but given the limitations of your hardware your options are somewhat also limited.