The people are
Vocaloids, computer-animated performers that use recorded voice software to sing
The title refers to
The last sound of their names, as pointed out in the Very Decisive Hint.
Oh, my beautiful lady first
This alludes to the fact that vocaloid software is made using the voices of real singers. The first hint references this, since although the singers are involved in the process of creating a vocaloid, they are not regarded as vocaloids themselves.
If today were not this blue
There would be a direct clue
These lines refer to the most famous vocaloid, Hatsune Miku, who is known for her blue hair and outfit.
Oh, please look at this mirror
Then you'll see me no error
If people called us twins
Then we will earn some wins
This verse is about two vocaloids: Kagamine Rin and Kagamine Len. They were designed to be "mirror images" of each other, but fans often refer to them as twins instead due to their similar appearance.
Oh, everything goes around
The meaningful another round
This one I'm not sure about, but this might be Megurine Luka. Her vocaloid software was originally intended to be used as the voice of Hatsune Miku, but was postponed until later, where she became her own character. Her surname in Japanese kanji is 巡音, the first character roughly translating to "round" and the second to "sound".