Do you want equivalent expressions or do you want to know how to use the given phrases? These seem like contradictory requests. Nevertheless...
A matter of time is not particularly formal. It's used in everyday conversation. But you should use It's in conversational English, remove the comma, and use the correct verb conjugation:
It's a matter of time before somebody finds it.
Note that somebody is correctly used rather than the somewhat more formal someone.
Again, as time goes by is not necessarily formal. But over time is probably less formal than it.
One problem is that you cannot use a past tense with as time goes by. So
*As time goes by, he found that his colleague is dishonest.
is ungrammatical, which is what the asterisk (*) before it means.
You can say as time went by with a past tense verb, but that is somewhat formal.
Over time he found that his colleague was dishonest.
Hope this helps!