Is it the standard or the Pro version of the Focusrite Voicemaster you are using?

The headphone output (at least on the Pro version) is dual mono, with normally the same signal sent to both ears. On the Pro, the ADC Ext input can feed the R channel of both the headphone output and the digital output while the Voicemaster processing is maintained the left channel. This allows two Voicemaster units to be used concurrently, the analogue output of the second device being fed to the ADC Ext input of the first, with the two separate signals sent to the L and R phones respectively.

Can you explain a bit more about your TRRS adaptor cabling? Plugging a TRS plug into a TRRS socket normally shorts the second ring (the second R) to the sleeve (the S). Most Galaxy phones have the second R as the ground and the S as the microphone input (with 3V plug-in-power on it), so standard TRS phones work correctly and ground the microphone input.

I'm assuming your "splitter" cable routes the T of its (mono) microphone input jack to the S on the adaptor cable, using the cable's second R as the ground for both the headphone and the microphone jacks. However, there are two types of splitter cable. The L and R headphone signal is the same in both, but in the older type of cable, the microphone is on the second R and the ground on the sleeve, and in the more modern cable, these last two are interchanged. It's worth buzzing this through to check which you have. If it's the wrong type, it would explain why you get no signal through from the Voicemaster.

Two further things to be aware of about the microphone input: (1) It's expecting an input level in the tens of millivolts, so the native Voicemaster output is almost certain to overload it. The use of attenuation resistors is an easy solution for this. (2) The microphone input normally has the plug-in power enabled. If your phone has an option to turn this off, do that, otherwise use a medium value, low voltage capacitor in series with the input line (e.g. 10uF/6.3V).

I hope the audio quality of the microphone input on your Galaxy phone is better than on the one I have, which is an older model.

Thanks for the reply,

I managed to get it working by using the +4dBV Output on the Focusrite Voicemaster Pro via an XLR to 3,5mm jack cable which then goes through the TRRS to 2x TRS splitter (OMTP) and finally to the TRRS to USB-C Adaptor. What a hassle. But got a very crisp voice on the recording now.

I'm glad you got it working, although I'm surprised you had to use an OMTP splitter rather than a CTIA. I had thought that all Samsung Galaxy phones had the CTIA pin configuration on their headset jacks.

One way of explaining this is could be that the OMTP splitter cable is giving you mono microphone input on the 3.5mm jack sleeve and ground on the jack tip. This connection would give the appearance of working (but with a polarity inversion) as long as your phone is not grounded to anything else. Does the audio input signal disappear when you short together the microphone jack sleeve and the jack sleeve on the headphone split?

How is the XLR - 3.5mm jack plug cable wired? Is it just picking out pin 2 of the XLR and sending that to the plug tip? Are you using anything to reduce the signal level from the Focusrite output?