@Dameo – so the splitter does NOT solve your volume problem, it will just split the audio from the HDMI source and leave you with the same problem. What you want to seek out is a volume controller, preferably an analog volume controller.
What TVs and computers often do are to send a full signal out of the device. What that means is it is super loud. And as you figured out, the volume control does not change the signal.
Instead of getting the TOSLINK to Aux, I would have recommended getting a decent quality Digital to Analog Converter that has TOSLINK and Coax inputs. If you buy a good one, like some of the Topping, they come with a remote and the ability to change the volume. That would solve your issue instantly.
Basically, similar to car audio, you want the signal turned as high as possible without clipping. You then have one unit, the head unit, control the volume. In home audio, they expect you to go from your devices, like a TV, to a receiver or pre-amplifier, which then either gets processed there or volume control applied, then sent to the internal amp for a receiver or to the external amplifier, then onto the speakers.
What normally happens is you have the digital signal go through the digital to analog converter, giving an analog audio signal that is usable. From here, usually you send it to a pre-amplifier. This allows the signal to be controlled, meaning volume alteration. From there, it is fed to the amplifier.
In many receivers, they also have HDMI decoders and act as HDMI switches, that way everything plugs into them and you use them to select the input source. The audio is decoded, then sent to the speakers (either through pass through out if available on the receiver or the receiver acts as the pre-amp and volume control, then sends it to the internal amplifiers, then out to the speakers).
So, you have choices. You can either buy a pre-amp or you can buy a volume controller.
Here are some examples of volume controllers:
Now, to make it more complicated, you should look for a volume controller that is analog. Why? Because of Signal to Noise Ratio (SNR). I’ll be writing an article on this soon. Basically, digital processing of volume, if at 16-bit, will greatly reduce the SNR, meaning that it can effect the quality of the sound at low volumes. This depends on the device used, etc. If it has 32-bit processing and it has direct access to the audio path, then the 32-bit would be fine with very low reduction in SNR (still researching this and implementations into DACs). But, analog volume controls will keep the SNR practically intact. That makes it the best option. However, you then are limited to the SNR of the analog volume controller (or the lowest SNR in the chain, often is the amp, but it can be elsewhere). You just need to make sure that the SNR of the other devices are higher than the amplifier to give very clear production. Otherwise, the noise floor would be that of the processors in the chain.
I know, complicated. But I hope this helps.
If instead of buying something like those you decide for a DAC, just pay attention to if it has single ended or balanced outputs and what the input to your amplifier unit is. If it is simple L+ and ground, that is single ended. If you have L+ gnd L-, you are looking at a balanced input for the amplifier and I would recommend finding a DAC with balanced out.
If you have any more questions, feel free to ask.