Android MediaPlayer won't play music on API 26+

You need to define a res/xml/network_security_config.xml and permit HTTP for that host:

<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
    <domain-config cleartextTrafficPermitted="true">
        <domain includeSubdomains="true">server.com</domain>
    </domain-config>
</network-security-config>

That network_security_config.xml also needs to be referenced in the Manifest.xml:

<?xml version="1.0" encoding="utf-8"?>
<manifest
    ...>
    <application
        android:networkSecurityConfig="@xml/network_security_config"
        ...>
            ...
    </application>
</manifest>

The SDK documentation explains it all in detail - and this applies to all network traffic an app creates.

Instead of lowering the security standards, upgrading the connection to HTTPS should be preferred.

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And concerning that one deprecated method .setAudioStreamType(), use .setAudioAttributes() instead - as your code already does. It does not seem to be the "main problem" here; verified that.