在Android Q中，可以使用MediaMuxer类来合并音频和视频文件。以下是一个示例代码：

import android.media.MediaCodec;
import android.media.MediaExtractor;
import android.media.MediaFormat;
import android.media.MediaMuxer;

import java.io.IOException;

public class MediaUtils {

    public static void mergeAudioVideo(String audioPath, String videoPath, String outputPath) {
        MediaMuxer mediaMuxer = null;
        MediaExtractor audioExtractor = null;
        MediaExtractor videoExtractor = null;

        try {
            mediaMuxer = new MediaMuxer(outputPath, MediaMuxer.OutputFormat.MUXER_OUTPUT_MPEG_4);

            // Extract audio data
            audioExtractor = new MediaExtractor();
            audioExtractor.setDataSource(audioPath);
            int audioTrackIndex = selectTrack(audioExtractor, "audio/");
            audioExtractor.selectTrack(audioTrackIndex);
            MediaFormat audioFormat = audioExtractor.getTrackFormat(audioTrackIndex);
            int audioTrack = mediaMuxer.addTrack(audioFormat);

            // Extract video data
            videoExtractor = new MediaExtractor();
            videoExtractor.setDataSource(videoPath);
            int videoTrackIndex = selectTrack(videoExtractor, "video/");
            videoExtractor.selectTrack(videoTrackIndex);
            MediaFormat videoFormat = videoExtractor.getTrackFormat(videoTrackIndex);
            int videoTrack = mediaMuxer.addTrack(videoFormat);

            mediaMuxer.start();

            // Write audio data
            ByteBuffer buffer = ByteBuffer.allocate(64 * 1024);
            MediaCodec.BufferInfo bufferInfo = new MediaCodec.BufferInfo();
            bufferInfo.presentationTimeUs = 0;

            while (true) {
                int sampleSize = audioExtractor.readSampleData(buffer, 0);
                if (sampleSize < 0) {
                    break;
                }

                bufferInfo.offset = 0;
                bufferInfo.size = sampleSize;
                bufferInfo.flags = audioExtractor.getSampleFlags();
                bufferInfo.presentationTimeUs = audioExtractor.getSampleTime();

                mediaMuxer.writeSampleData(audioTrack, buffer, bufferInfo);
                audioExtractor.advance();
            }

            // Write video data
            buffer.clear();
            bufferInfo = new MediaCodec.BufferInfo();
            bufferInfo.presentationTimeUs = 0;

            while (true) {
                int sampleSize = videoExtractor.readSampleData(buffer, 0);
                if (sampleSize < 0) {
                    break;
                }

                bufferInfo.offset = 0;
                bufferInfo.size = sampleSize;
                bufferInfo.flags = videoExtractor.getSampleFlags();
                bufferInfo.presentationTimeUs = videoExtractor.getSampleTime();

                mediaMuxer.writeSampleData(videoTrack, buffer, bufferInfo);
                videoExtractor.advance();
            }

            mediaMuxer.stop();
            mediaMuxer.release();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (audioExtractor != null) {
                audioExtractor.release();
            }

            if (videoExtractor != null) {
                videoExtractor.release();
            }

            if (mediaMuxer != null) {
                mediaMuxer.release();
            }
        }
    }

    private static int selectTrack(MediaExtractor extractor, String mimeType) {
        int numTracks = extractor.getTrackCount();
        for (int i = 0; i < numTracks; i++) {
            MediaFormat format = extractor.getTrackFormat(i);
            if (format.getString(MediaFormat.KEY_MIME).startsWith(mimeType)) {
                return i;
            }
        }
        return -1;
    }
}

使用上述示例代码，你可以调用mergeAudioVideo方法来合并音频和视频文件。其中，audioPath是音频文件的路径，videoPath是视频文件的路径，outputPath是合并后的文件输出路径。