This is a simple program to convert regular files into audio files which can be played into the cassette input on an Apple ][ to transfer data to it.
The output format is a single channel pcm file with U8 samples and a sample rate of 44100 kHz.
cc -lm encode.c -o encode
./encode file -o file.pcm
If no input or output file is given, they default to stdin and stdout
respectively.
Since the output is raw data, you have to specify the format yourself.
You could convert to some other format with a program like audacity,
but I've been using aplay...
aplay -c1 -fU8 -r44100 file.pcm