So you've downloaded a track and you can now hear all other participants on that track despite the fact that you were all wearing headphones.
If this is already done you may be able to recover this track by using the new Magic Compose tool (to add echo cancellation) which should show as an option at the bottom of each recording session box on the View All Recordings page.
There are several things that can cause this such as
crosstalk (https://en.wikipedia.org/wiki/Crosstalk) and
audio spill/bleed/leakage (https://en.wikipedia.org/wiki/Spill_(audio).
This can be caused because of some ways your setup may be functioning differently whilst recording through a computer and browser-based app.
Headphones plugged into a USB microphone
If your/your guest's headphones are plugged into a USB microphone and your microphone gain/volume is high this can cause other participants to be recorded on that track.
To avoid this in future, it is best to ensure that headphones are plugged directly into your computer/device.
If this isn't possible attempt to reduce your gain.
Lastly, you can enable Echo Cancellation on Riverside.fm to attempt to negate this but please keep in mind that this can degrade the audio quality.
Using earbuds plugged into an AUX/headphone cable port
If your guest is using a Mac and has used a set of earbuds the Mac will try to set the headphones in the earbuds as the default but this can cause this audio issue.
To remedy this please ensure that your participant has the (Built-in) microphone that lists their device first such as MacBook Pro Microphone (Built-in) as per the example below and not the External Microphone (Built-in).