"how can I preserve the previous data associated with this plugin?" – If the plugin worked on the 3.5.1 site and you have a backup of that site – both database and Moodledata – then I would suggest keeping that as an archive copy of the site, not generally available to users but where old submissions can be retrieved from.
"is there any replacement"? – Using online text submissions students can use the Record Audio button in the text editor:
