要将立体声编码为单声道的MP3，可以使用Android的MediaCodec和MediaMuxer类来实现。以下是一个代码示例：

import android.media.MediaCodec;
import android.media.MediaCodecInfo;
import android.media.MediaCodecList;
import android.media.MediaExtractor;
import android.media.MediaFormat;
import android.media.MediaMuxer;
import android.os.Build;
import android.os.Environment;
import android.util.Log;

import java.io.IOException;
import java.nio.ByteBuffer;

public class StereoToMonoConverter {
    private static final String TAG = "StereoToMonoConverter";

    public static void convertToMono(String inputFilePath, String outputFilePath) {
        MediaExtractor extractor = new MediaExtractor();
        MediaMuxer muxer = null;
        MediaCodec codec = null;

        try {
            extractor.setDataSource(inputFilePath);

            int audioTrackIndex = getAudioTrackIndex(extractor);

            if (audioTrackIndex >= 0) {
                MediaFormat inputFormat = extractor.getTrackFormat(audioTrackIndex);
                String mimeType = inputFormat.getString(MediaFormat.KEY_MIME);

                MediaFormat outputFormat = createOutputFormat(inputFormat);

                muxer = new MediaMuxer(outputFilePath, MediaMuxer.OutputFormat.MUXER_OUTPUT_MPEG_4);

                codec = MediaCodec.createEncoderByType(mimeType);
                codec.configure(outputFormat, null, null, MediaCodec.CONFIGURE_FLAG_ENCODE);

                extractor.selectTrack(audioTrackIndex);
                codec.start();

                ByteBuffer[] inputBuffers = codec.getInputBuffers();
                ByteBuffer[] outputBuffers = codec.getOutputBuffers();

                MediaCodec.BufferInfo bufferInfo = new MediaCodec.BufferInfo();
                boolean isEOS = false;

                while (!isEOS) {
                    int inputBufferIndex = codec.dequeueInputBuffer(10000);
                    if (inputBufferIndex >= 0) {
                        ByteBuffer inputBuffer = inputBuffers[inputBufferIndex];
                        int sampleSize = extractor.readSampleData(inputBuffer, 0);

                        if (sampleSize < 0) {
                            codec.queueInputBuffer(inputBufferIndex, 0, 0, 0, MediaCodec.BUFFER_FLAG_END_OF_STREAM);
                            isEOS = true;
                        } else {
                            long presentationTimeUs = extractor.getSampleTime();
                            codec.queueInputBuffer(inputBufferIndex, 0, sampleSize, presentationTimeUs, 0);
                            extractor.advance();
                        }
                    }

                    int outputBufferIndex = codec.dequeueOutputBuffer(bufferInfo, 10000);
                    switch (outputBufferIndex) {
                        case MediaCodec.INFO_OUTPUT_FORMAT_CHANGED:
                            MediaFormat newFormat = codec.getOutputFormat();
                            int trackIndex = muxer.addTrack(newFormat);
                            muxer.start();
                            break;
                        case MediaCodec.INFO_OUTPUT_BUFFERS_CHANGED:
                            outputBuffers = codec.getOutputBuffers();
                            break;
                        case MediaCodec.INFO_TRY_AGAIN_LATER:
                            break;
                        default:
                            ByteBuffer outputBuffer = outputBuffers[outputBufferIndex];
                            muxer.writeSampleData(trackIndex, outputBuffer, bufferInfo);
                            codec.releaseOutputBuffer(outputBufferIndex, false);
                            break;
                    }
                }

                codec.stop();
                codec.release();
                extractor.release();
                muxer.stop();
                muxer.release();
            }
        } catch (IOException e) {
            Log.e(TAG, "Failed to convert stereo to mono: " + e.getMessage());
        }
    }

    private static int getAudioTrackIndex(MediaExtractor extractor) {
        int numTracks = extractor.getTrackCount();
        for (int i = 0; i < numTracks; i++) {
            MediaFormat format = extractor.getTrackFormat(i);
            String mime = format.getString(MediaFormat.KEY_MIME);
            if (mime.startsWith("audio/")) {
                return i;
            }
        }
        return -1;
    }

    private static MediaFormat createOutputFormat(MediaFormat inputFormat) {
        MediaFormat outputFormat = MediaFormat.createAudioFormat(MediaFormat.MIMETYPE_AUDIO_MPEG, inputFormat.getInteger(MediaFormat.KEY_SAMPLE_RATE), 1);
        outputFormat.setInteger(MediaFormat.KEY_BIT_RATE, inputFormat.getInteger(MediaFormat.KEY_BIT_RATE));
        outputFormat.setInteger(MediaFormat.KEY_AAC_PROFILE, MediaCodecInfo.CodecProfileLevel.AACObjectLC);
        outputFormat.setInteger(MediaFormat.KEY_MAX_INPUT_SIZE, 16384);

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            outputFormat.setInteger(MediaFormat.KEY_CHANNEL_MASK, AudioFormat.CHANNEL_OUT_MONO);
            outputFormat.setInteger(MediaFormat.KEY_CHANNEL_COUNT, 1);
        } else {
            outputFormat.setInteger(MediaFormat.KEY_CHANNEL_COUNT, 1);
        }

        return outputFormat;
    }
}

使用示例：

String inputFilePath = "/path/to/input/stereo.mp3";
String outputFilePath = "/path/to