For those who are too lazy to unlock it themselves, here’s how:

Make your arcade rivals “BEAT”, “MANIA”, “IIDX”, “16″, “SECRET”
Make an original expert course and call it “BEATFREE,” the contents do not matter
Play through it, let the credits roll
Have an enjoy.

VN:F [1.7.2_963]Rating: 3.7/5 (3 votes cast)

Here are what we believe to be the final unlocks for Sirius AC. Plus ratings.
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[BUBBLEGUM DANCE] Ryu* – bass 2 bass SP:4/7/10 DP:5/7/10
[EUROBEAT] Y&Co feat. Erika – IN THE NAME OF LOVE SP:4/7/9 DP:4/8/10
[J-HOUSE] kors k feat. Suzuyo Miyamoto – Special One SP:5/7/9 DP:5/8/9
[HARD RENNAISANCE] dj YOSHITAKA – Evans SP:6/10/12 DP:7/10/12
[...]

The upcoming Jubeat Ripples Append is said to link with beatmania IIDX 17 somehow, and someone fishing through stuff has discovered that yes, there are hidden Jubeat songs on the Sirius hard drive that have not been unlocked yet. The person supplying this information only revealed 4 of them, but there is allegedly one more.
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We seem to have received confirmation that the extra stage system on Sirius is now becoming accessible to players, here’s what we know:
Here’s what we know:

Once you get in, you need to collect keys for each song to unlock the extra boss song on each folder (on Normal, easy clear, Hyper, AA, and on Another, [...]

It was better than Sbemaliarized Entertainment, for sure.

Now you believe us?