Audacity can help you to add envelopes; it's got a dedicated tool!
the audacity manual page on the envelope tool: https://manual.audacityteam.org/man/envelope_tool.html
basic how-to with the envelope tool: https://matthewsetter.com/audacity-envelope-tool/
great practical explanation and demonstration: https://www.youtube.com/watch?v=wP9bzByrE0I