Applies To Platinum: VoiceMaster Pro, TrakMaster Pro
The Platinum VoiceMaster Pro is a mono unit i.e. it only has one channel.
SPDIF is a stereo digital audio format i.e. it has two channels (left and right). The signal from the VoiceMaster Pro appears on the left channel of the SPDIF signal only. This is done so that the right channel of the converter is free to convert another source e.g. another preamp, to the digital domain (via the 'ext. A/D input').
When you record into a DAW using the VoiceMaster Pro you need to record onto a mono channel, not a stereo channel. Make sure the input for that channel is set to SPDIF Left and that the track is panned centre in the mixer.