This is my contraption used to connect my phone to the TI-99/4A cassette (CS1) port, allowing a WAV or MP3 file containing TI program data to be loaded to the TI. The phone must recognize the cable as being a line-level device rather than headphones, otherwise it attenuates the audio to a point where it is barely audible. The two-to-one cable is necessary as both channels are needed for the TI to "hear" the audio. This is a passive device simply connecting the RCA pins to the appropriate cassette port pins, and I have given consideration to building in a low-power amp using the cassette motor line for power.
This cable could be also be used for devices with line-out other than a cell phone.
The audio files used (WAV or MP3) can be created by loading a TIFiles program into a program like CS1er or Tape994A and saving is WAV. I suppose just about any MP3 encoder could be used, though I use lame's command line program to convert to VBR (variable bit rate) using the --vbr-new option. You might be able to use lame to make adjustments, but I use SOX (SOund eXchange) to make minor tweaks in volume when necessary. You can usually get a 41% increase in volume without clipping. (I have not tried this, but apparently newer versions of SOX support MP3 output.)