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"?>
<manifestxmlns: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-screensandroid:smallScreens="true"android:normalScreens="true"android:largeScreens="true"android:xlargeScreens="true"android:anyDensity="true"/><applicationandroid: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*

.

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。