Nokia Lumia 521 - After installing the latest Audio app update in my Lumia 520, 521, 525 or 526 phone, the equaliser menu is briefly shown when starting the app. Is equaliser supported in my phone?

No, the equaliser is not supported in Lumia 520, 521, 525 or 526 phones. The audio app version 2.1.1.1 (released March 2014) shows briefly the equaliser menu when starting the app but this doesn't mean that the equaliser support is included in these Lumia models. The equaliser menu is shown for a short moment after which it disappears and the correct audio menu for Lumia 520, 521, 525 and 526 is shown. This is normal behaviour and there's no fault in the phone.