There are some C1P WAV files here:
https://osi.marks-lab.com/software/games.html (But they all expect a 1K screen)
Otherwise, if you have a .lod file you want to use, drop it onto the OSIKCS.EXE file in Windows Explorer and you'll have a 300baud WAV file to play with.
The problem you'll have with the C2/C4 files is many use wide screen mode where only the 1st 32 characters per row are visible (visually similar to C1P native screen), also the keyboard values are inverted between the C1 and C2/C4 systems so programs that access the keyboard directly will need to be modified. This is a common activity when porting programs between the two systems.
You may find some UK101 programs work as they use the same keyboard interface as C1P, and some of them were written for a 2K screen. However the UK101 uses a different character generator ROM, so some displayed symbols may be incorrect. Also some programs require functions available in a specific monitor ROM (like Cegmon or Newmon) which will not work with SYN600. UK101 BASIC removes a restriction on the symbols you can load from tape although it is otherwise nearly identical to OSI BASIC, so some programs will give you a syntax error on OSI BASIC that would be accepted with UK101 BASIC.
You can use WinOSI to load files from diskette images, then use the File/Decode Basic... menu to extract the BASIC code, save to a txt file & run through OSIKCS to get a tape loadable file. Many of the Aardvark text adventures should work, although they may be nearly 8K in size which require a looong time to load at 300baud! Other bog standard BASIC program should work fine.
Is there a particular program you are interested in?
Edit: [I see this topic was updated while typing -- Doh!]
Re: OSIKCS There is actually code in the program commented out which changes the WAV header to 21600 sample/sec which then generates perfectly compressible 1200/2400 300-baud tones - at around offset 25-30 in the WAV file change two instances of hex 22 56 to 60 54
The 22050 value was used for historical purposes and was the sample rate used by Ed's CUTS program. The old Sound Blaster had fixed sample frequencies it could deal with and the slightly faster playback didn't seem to affect OSI loads operations. What OSIKCS really needs is a lowpass filter on input and some logic work to make decoding better.