We can customize the sound with a 3rd party email app but it's a paid app (not free). The Profimail app, using its Rules feature... if the "To contains (your email1) then play (this alert1)". Add multiple rules for multiple domains/email addresses. We can also make a different rule if the email is from a certain person... "From contains (email9) then play (this alert9)" or show in a different color, etc. However the alert/sound part becomes a mess when multiple emails in multiple accounts or from multiple people on the special rules arrive at the same time
The Rules in Profimail is actually a very powerful feature - i also use it to delete (or mark for delete) spams with certain keywords in the Subject/Title line, etc. Only two issues... Profimail is not compatible with Exchange server (use Moxier/etc) and the email's HTML rendering is not pretty (forced/strange formatting) but all the graphics/text is there.