测试版本为Unity2017.4.25f1

1.在自定义AndroidManifest.xml（位于Assets\Plugins\Android\）中添加android:exported="true"

<?xml version="1.0" encoding="utf-8"?>
<manifest
    xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.unity3d.player"
    xmlns:tools="http://schemas.android.com/tools"
    android:installLocation="preferExternal"
    android:versionCode="1"
    android:versionName="1.0">
    <supports-screens
        android:smallScreens="true"
        android:normalScreens="true"
        android:largeScreens="true"
        android:xlargeScreens="true"
        android:anyDensity="true"/>
    <application
        android:theme="@style/UnityThemeSelector"
        android:icon="@drawable/app_icon"
        android:label="@string/app_name"
        android:requestLegacyExternalStorage="true">
        <activity android:name="com.unity3d.player.UnityPlayerActivity"
                  android:label="@string/app_name"
                  android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
            <meta-data android:name="unityplayer.UnityActivity" android:value="true" />
        </activity>
    </application>
  <uses-permission android:name="android.permission.ACCESS_WIFI_STATE"></uses-permission>
  <uses-permission android:name="android.permission.CHANGE_WIFI_STATE"></uses-permission>
  <uses-permission android:name="android.permission.INTERNET"></uses-permission>
  <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"></uses-permission>
  <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION"></uses-permission>
  <uses-permission android:name="android.permission.WRITE_SETTINGS"></uses-permission>
  <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
  <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
  <uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS" />
</manifest>

2.在菜单File>Build Settings...>PlayerSettings...>Other Settings，勾选Mute Other Audio Source*

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3.通过以上两步应该就可以在安卓12上运行了。如果还不行就加上签名再试试。