This app was mentioned in 10 comments, with an average of 1.50 upvotes
https://play.google.com/store/apps/details?id=com.njsoftware.volumechanger&hl=en
There are several similar apps. I realize now that they don't have sliders.
Best next thing is to install Edge Gestures and control the volume by swiping on the edge of the screen.
Try "Volume in notification". https://play.google.com/store/apps/details?id=com.njsoftware.volumechanger It's mainly designed to give quick buttons in a drop-down menu to mute or increase/decrease the media volume, but as a side-by-side it it also can also show a muted/injured indicator in the taskbar all the time.
Or maybe this? https://play.google.com/store/apps/details?id=com.njsoftware.volumechanger
This app capable of only increase/decrease the media's volume.
Edit: sadly it works differently, not like a notification sidebar.
https://play.google.com/store/apps/details?id=com.njsoftware.volumechanger for notification control
https://play.google.com/store/apps/details?id=il.co.anykey.apps.floatingvolume for a floating app
In the extremely unlikely case that the physical buttons break during the lifetime of the phone, you could download an app that allows you to adjust volume through software from the notification tray. Like this: https://play.google.com/store/apps/details?id=com.njsoftware.volumechanger
That doesn't steal the focus from the current app.
I was a Pro Android, and specifically a Samsung user for a long time. I recently switched to an old iPhone 5 from a Galaxy S4 and I have to say, I miss a few things, but over all I've taken a huge foot in the mouth for all the bad I used to say about iOS.
Bloatware Something you might not expect coming from iOS is all of the bloatware. This is especially the case on Samsung devices, and although the S6 claims to have less of it, I've seen it and personally I don't see a difference. Many say you can just sweep it under the rug and won't ever notice it again, but I believe my bloatware caused noticeable battery drain. You can disable much of it, but not always everything. Also, you better buy a higher storage option because the bloatware isn't light in storage usage.
Battery Battery life is controversial, but one thing that is for certain is that standby time is definitely going to be better on an iPhone. Android is constantly running something in the background even when you've closed out tasks. You can leave an iPhone off a charger overnight and lose a few percent. Do that with an Android phone and it's 20-30 %. Whether or not that's important to you as almost everything charges their phones overnight is up to you. But it's nice knowing as long as I cut back using my phone that the battery will most likely last through until I can get to a charger.
TouchWiz I have to say, TouchWiz sucks. I couldn't stand the look of it, and the fact that it hinders performance and is a RAM hog when you compare it to vanilla Android. I would say go for something else based solely on the fact that TouchWiz kinda ruined Android for me in certain aspects. (See below)
Fluidity & Ease of use Once I switched to iOS it was a breath of fresh air honestly. Far far less were random lags and app crashes happening. The bottom swipe up area is genius IMO. With Android devices getting bigger and bigger as a standard, it makes no sense as to why we're still straining to swipe from the top to reach functions that need to be accessed quickly. I didn't notice this until using an iPhone. Many say the widgets on iOS are silly and less competent versions of Android's. But I have to say, widgets on Android come with a price, you can't have a page full of widgets and expect your battery to last forever. I like that on iOS they're all in one clean designated area, and I don't believe that they update unless you are actually using them. Maybe I changed, but I really only found myself using widgets that I needed to use because of Android (RAM clearer, task killer)
Volume control OMG Controlling media volume (videos, music, etc.) has always been less intuitive in Android than in iOS. But with the latest brainchild that is Android Lollipop, the geniuses at Google took it a step further. It used to be that you can hit the volume rocker and it would (annoyingly so) change your ringtone volume a notch, but would pop up a little volume mixer icon next to the slider for the ringtone volume so that you could tap it and then pullup a little window to change the volume of everything else (Media, notifications, alarms). Well they killed that function in Lollipop. Now the ONLY way to change media volume, is to either go all the way into your settings, or take a gamble that the last video or song you listened too was on lower volume, and then pound the volume down button to prevent an embarrassing situation (which doesn't always work well due to Androids lag, causing the phone to not detect you're watching a video, so it starts changing ringtone volume instead of media). Or option 3, download a really ugly notification widget , which, by the way, has to use buttons instead of a slider because Android doesn't allow a slider in the notification area.