what new in ios 6.1.2? is it worth

90% sure this is software issue :-). There are several possible solutions that have worked for others including myself in the different forums.

What worked for me: Plug in headphones and start a song (Music) or audio stream (TuneIn Radio). While listening to the song/audio with headphones with the app open, unplug your headphones. On my phone the volume bar immediately became visible and I have not had any issues since. I can now listen to music, play games with audio, etc., without my headphones and the volume works just fine.

What worked for others:

Solution 1: Go to Music, start a song (leave it playing) then go to Settings, turn on Airplane mode, go back to Music and look for volume bar. If it appears you should be good. Then go turn off Airplane mode.

Seems the Volume bar just needs a jump start...after that it appears to work...again, definitely stinks of a software glitch. My wifes iPad 4 does something similar where the Volume randomely stops working. With her iPad a 'hard' reset (hold power and main button down 'til you see the Apple) works to restore sound.

Is yours 4s? Have you recently updated to 6.1 or 6.1.2?

Looks like apple is kiding with us. Ive had this problem since Ios 6.1 and now after 3 days I ulgraded to 6.1.2 this ****** issue has returned. Again the other part will listen me if I scream HELLOOo in the first second and then everything works fine or if I wait 1 minute but none can wait that.

Im really angry with this company always they mess up after each update

What are we gonna do folks??