Learn English – “If it helps you” or “that helps you”

The two phrases have the same basic meaning, but native speakers tend to use the "one hundred percent sure" wording in order to to give a greater impression of certainty. In speech, you can emphasise the word "one" more strongly than "a" and the phrase is spoken with a regular rhythm as ONE HUNdred perCENT SURE.

Both sentences essentially mean the same thing, that someone was singing and you heard they, and often them can be used interchangeably.

I heard him sing.

is more general, nonspecific, and "passive".

I heard him sing in La Traviata.

The more "active"

I heard him singing

can mean the same, but may leave the listener asking "Singing what? It is usually followed by a description of what he was singing

I heard him singing a happy song.
I heard him singing to the birds.