There is Photosounder which have excellent sound quality (all edit is in frequency domain and it then convert back) For free there is also Virtual ANS and http

There is Photosounder which have excellent sound quality (all edit is in frequency domain and it then convert back) For free there is also Virtual ANS and https://www.fsynth.com https://www.fsynth.com on the more experimental side (conversion is done raw using additive synthesis, phase information is lost so sound quality is affected)