实测音频大小

录音1分钟：
caf格式用了2.6MB
mp3格式用了227KB 

录音10分钟：
caf格式用了26.5MB
mp3格式用了2.3MB

操作流程

1.导入lame gitbub地址： https://github.com/hedgehogIda/caf-mp3

2.声明头文件

#import "lame.h"

3.实现代码我的博客中实现录音功能的这篇文章，里面有获取tmpUrl的代码。 https://www.jianshu.com/p/fea0cbe42cb2

/**
 caf转MP3
 */
- (void)conventToMp3 {
    //tmpUrl是caf文件的路径，并转换成字符串
    NSString *cafFilePath = [tmpUrl absoluteString];
    //存储mp3文件的路径
    NSString *mp3FilePath = [NSString stringWithFormat:@"%@.mp3",[NSString stringWithFormat:@"%@",[cafFilePath substringToIndex:cafFilePath.length - 4]]];
    @try {
        int read, write;

        FILE *pcm = fopen([cafFilePath cStringUsingEncoding:1], "rb");  //source 被转换的音频文件位置
        fseek(pcm, 4*1024, SEEK_CUR);                                   //skip file header
        FILE *mp3 = fopen([mp3FilePath cStringUsingEncoding:1], "wb");  //output 输出生成的Mp3文件位置

        const int PCM_SIZE = 8192;
        const int MP3_SIZE = 8192;
        short int pcm_buffer[PCM_SIZE*2];
        unsigned char mp3_buffer[MP3_SIZE];

        lame_t lame = lame_init();
        lame_set_in_samplerate(lame, 11025.0);
        lame_set_VBR(lame, vbr_default);
        lame_init_params(lame);

        do {
            read = fread(pcm_buffer, 2*sizeof(short int), PCM_SIZE, pcm);
            if (read == 0)
                write = lame_encode_flush(lame, mp3_buffer, MP3_SIZE);
            else
                write = lame_encode_buffer_interleaved(lame, pcm_buffer, read, mp3_buffer, MP3_SIZE);

            fwrite(mp3_buffer, write, 1, mp3);

        } while (read != 0);

        lame_close(lame);
        fclose(mp3);
        fclose(pcm);
        NSLog(@"转换成功");
    }
    @catch (NSException *exception) {
        NSLog(@"%@",[exception description]);

    }
    @finally {

    }
}