The best audio tester for your song recordings is your phone
...but not for the reason you may think.
While true that phones are getting better with audio all the time, the one thing that will absolutely never sound right is the crappy tiny handset speaker. No matter what, that thing will always sound awful.
And it just happens to be that chintzy speaker is in fact exactly what you want when testing out your music recordings.
The standard advice that used to be given to anybody who made music recordings was to use a stock car stereo for audio testing. The reasoning behind this is that car stereos for a very long time were terrible. Most cars and trucks used to be delivered from the manufacturer with a stereo that, at best, only delivered 30 watts. All you had for sound shaping options was a 2-band EQ (treble and bass) and maybe at loudness on/off toggle. The speakers were also seriously cheap and would blow out easily as well.
Garbage car stereos made for great audio testing environments, because if your recorded song sounded good there, it would sound good anywhere since just about every other stereo sounded better than what was in most cars.
This advice no longer applies because even the cheapest cars now have sound systems that sound a million times better than what car manufacturers used decades ago.
Crappy smartphone speakers to the rescue
The handset speaker on your phone has a crackly nasty sound and distorts very easily when you increase the volume even moderately. It doesn't matter what phone you have because they all sound terrible. The speaker is tiny, and because of its size and the fact it's in such a slim enclosure means no bass response whatsoever.
Nobody likes the sound of a smartphone handset speaker - but - like with those old crappy car stereos, if your audio recording sounds good on that tiny terrible thing, it will sound good on any other system. And by "good" I mean "good enough".
What should you listen for when testing?
It's not so much what you hear when you test your music recordings on the phone, but rather what you don't.
A telltale sign your mix isn't quite correct is if certain things "disappear" from your song when listening to it on a smartphone speaker. Hearing no bass is a classic example of this, but the fix is not to make the bass louder. Instead, you increase the midrange frequencies of the bass track instead. Once the bass is to a point where it can be heard on a smartphone speaker, even if only a little bit, then at least your song sounds closer to what you originally intended.
Partially passing the test is the goal
Will you ever get a "perfect mix" when playing back your recorded song on a phone speaker? No. The goal is, as said above, to make it sound good enough.
The phone's handset speaker is the worst sounding audio environment. I honestly can't think of anything people use that sounds worse. This being true, as long as your song sounds somewhat right on that tiny terrible speaker where you can hear mostly everything you want to, job done.
Making a song sound good on computer desktop speakers or ear buds is easy. But getting the song sounding partially right on a phone's handset speaker? Definitely more of a challenge, but one worth taking on.