Can you mix a single stereo file?
No. Not with Bandmixr, and not with a $200-an-hour engineer who has thirty years of credits either. That's just what the word mixing means.
Why a single stereo file can't be mixed
Mixing is balancing separate tracks against each other. The vocal comes up 2 dB. The rhythm guitars pan left and right. The bass gets a small cut around 60 Hz so the kick can punch through. Every one of those moves changes one instrument relative to the others, so every one of them needs the instruments to exist as separate files.
A stereo bounce has no others. Everything your band played is already summed into two channels, left and right. The kick, the bass, the vocal and the guitars are not stacked inside the file waiting to be picked apart. They are the same waveform now. Even the Audacity manual puts it plainly: once audio has been mixed down, it is essentially impossible to separate out the original parts again. They compare it to trying to take the banana out of a banana milkshake.
If the word stems is new to you, What Are Stems and Why Do They Matter covers the basics in five minutes.
Stereo mixdown vs stems: the Photoshop analogy
Think of a Photoshop file with layers versus a printed photo. With the layers, you can move the drummer, brighten just the singer, delete the amp buzzing in the background. With the print, you can make the whole image warmer or brighter, and that is all. Nobody, at any price, can move the drummer in a print.
A multitrack session is the layered file. A stereo mixdown is the print.
There is a quick tell hiding in how services are built, too. Everything that genuinely mixes asks you for more than one file. Bandmixr takes up to 16 stems on the free plan and 24 on paid. Services like LANDR and eMastered take one stereo file per song. Even if you feed them stems, each file just gets processed on its own; nothing balances the instruments against each other, because they are mastering services, whatever the homepage says. That split is the print versus the layers, expressed as an upload form.
What a stereo file can get: mastering
Your bounce is one step further along than you thought. If the balance inside it already works, what it needs is mastering. That's the finishing pass for an already-mixed song, where the whole thing gets its release loudness and a last bit of tonal polish. If the line between the two is fuzzy, Mixing vs. Mastering: What's the Difference and Why You Need Both draws it clearly.
Bandmixr has a path built for exactly this file. Open your project, click "Already got a finished mix? Click here for mastering". You skip the mixing step entirely. You get three presets: Streaming at about -14 LUFS for Spotify and Apple Music, Standard at about -11, and Loud at about -8. On paid plans you can also upload a reference track so the master leans toward a sound you already love. One thing to check before you upload: do not slam the bounce into a limiter first. How Much Headroom to Leave Before Mastering explains what the mastering stage needs from you.
Can mastering fix my mix? Mostly no
This is the honest part, and it is where a lot of services get vague. Mastering can make your song louder, brighter, warmer, more consistent. What it cannot do is fix balance, because every move hits the whole print at once. Boost 3 kHz to bring out the vocal and you also boost the snare, the guitar pick attack, and the cymbals, because they all live there too.
Working mastering engineers put a number on it. With mid/side processing, a centered vocal can be lifted by roughly 1 dB before the rest of the mix starts to warp. Their real answer to a buried vocal is not a clever plugin. It is "go back and remix." And clipping that got baked into the bounce stays baked in. Mastering will not remove it, and even dedicated repair tools can only sand down the damage, not restore what was lost.
So the 30-second answer: if your only complaint is that the song is quiet or dull next to released records, you don't need to mix an already mixed song, just master it. If your complaints are about instruments, the vocal is buried, the bass is a blur, or the drums just feel weak, then you need the separate tracks, and no amount of mastering will substitute.
Stem separation tools
Stem separation tools exist. They take a stereo file and estimate what the vocals, drums, bass and other instruments were. Bandmixr deliberately does not offer one, and it is worth saying why.
No separation engine produces clean stems from a finished mix. Vocals come out with a watery, phasey edge. Cymbals bleed into the vocal track. Kick and bass smear into one low-end blur, and dense rock recordings, exactly what most bands make, come out worst of all. Then a mix built from those stems inherits every artifact and bakes them in a second time. If the session file is genuinely lost and the song matters enough, separation is a last resort you can try elsewhere. If there is any way to get the real tracks, get the real tracks.
What to upload, in one sentence
If you have separate instrument tracks, upload the stems and let Bandmixr mix and master them; if all you have is one stereo file, skip straight to the mastering path and polish what you've got.