Looking forward to it is a more professional way to say sounds good when you’re speaking to your employer or if you are confirming an interview. Moreover, “on board” works well when we want to show someone that we have already made ourselves free and we like the idea they’ve put forward. The phrase “sounds good” is common in everyday conversations & informal communication like text messages. Just like the idiom time flies, it is not a literal saying! Sounds good can be used to signify:

You’ll use “Sounds great” when you wish to display acceptance or gratitude for a service or action someone has done for you. It will always be prompted by another assertion where someone notified you of a past, present, or future action. You use “Sounds great” to consent to the topic and nurture warm feelings from both or all people. It can be a grateful response to exciting news, such as the details of a job promotion or a trip planned home to visit family and friends. When Can You Use “Sounds Great”?Sounds like a plan” is a common informal phrase we use when someone has arranged an activity or event. Moreover, if you’re not too familiar with the person, this phrase works quite well. After all, it does not go overboard with enthusiasm but shows that you appreciate their effort toward making sure the plans are finalized.

I’m on board is a more casual and idiomatic phrase, so you should only use it if you have a friendly dynamic with your peers. Or, when speaking to a colleague you are close to.As a minor sentence, “Sounds great” functions as a complete thought, even without a definitive subject and verb ( source). A more detailed context will determine the specific connotation of this expression.