视频合并后第二个视频丢失声音。

需要使用FFmpeg库中的命令将第二个视频的音频流提取出来，然后将其与合并后的视频流再次合并。

代码示例：

import os

# 视频合并
def merge_video(video1_path, video2_path, merge_path):
    cmd = "ffmpeg -i {} -i {} -filter_complex \"[0:v] [0:a] [1:v] [1:a] concat=n=2:v=1:a=1 [v] [a]\" -map \"[v]\" -map \"[a]\" {}".format(video1_path, video2_path, merge_path)
    os.system(cmd)

# 提取音频并重新合并
def merge_audio(video_path, audio_path, merge_path):
    cmd = "ffmpeg -i {} -c copy -map 0:a {}".format(video_path, audio_path)
    os.system(cmd)
    cmd = "ffmpeg -i {} -i {} -codec copy -shortest {}".format(video_path, audio_path, merge_path)
    os.system(cmd)

# 调用函数进行视频处理
video1_path = "video1.mp4"
video2_path = "video2.mp4"
merge_path = "merge.mp4"
audio_path = "audio.mp3"
merge_video(video1_path, video2_path, merge_path)
merge_audio(merge_path, audio_path, merge_path)

注意事项：

1. 需要安装FFmpeg库。
2. 在提取音频和重新合并过程中需要保证音频和视频的格式一致。
3. 可以根据自己的需要调整视频和音频合并的方式，本代码示例采用了'concat”方法。