You taking us back to the early years of this century... the S03 came out around 2001 and was the smallest of the S-series synths.

The S80 (1999) was the 88-Key Modular Synthesis Plug-in System (top-of-the-line) fully editable Synth - that introduced SmartMedia card storage
The S30 (2000) was the 61-Key version of the S80 (MSPS compatible) fully editable Synth - SmartMedia card storage.

The S03 (2001) was a 61-Key lite version of the S30 (no expansion or storage media slots) this S-Series Synth was designed for those not looking to edit deeply; it featured the 480 sound Yamaha XG Library. Based on the Xpanded General MIDI soundset, you could edit any of these sounds using the protocol’s extensive ability to be programmed by a Sysex and Control Change commands in the GM Setup Measure.
This was followed in (2002) by an 88-Key version the S08.

The S80 and S30 were fully programmable. They had storage (SM card); expansion (PLG-plugin Boards)
The S08 and S03 were designed for those that wanted XG/GM compatibility with some ability to edit and store.

While you might find S80 and S30 sounds (on SmartMedia cards)
I find no sounds in my archives for the S03 or the S08; and I’m sure this because the nature of how you edited and stored sounds on this series. Your “edits” were typically fashioned by the MIDI file data in the Setup Measure of the XG File. Back then they shipped with a DAW called “XGworks” which had an XG editor built in ... if memory serves. I’m sure the last computer it ran on was probably Windows 98/ or Windows 2000 - don’t hold out any hope of finding and running that software. It connected to computers via an 8-pin MINI DIN to RS-323C (D-sub 9-pin) (yikes!)

Sorry to be the barer of bad news, but if you find an “XG Editor” you might find a way to create your own sounds for the S03... by editing the XG engine.
I do recall it had some method of creating and storing USER Voices on board, but I don’t recall any method of external storage other than MIDI bulk dump.