Class to convert AVAudioPCMBuffer to CMSampleBuffer.

Converter.swift

import Foundation
import AVFoundation
import CoreMedia

class Converter {
    static func configureSampleBuffer(pcmBuffer: AVAudioPCMBuffer) -> CMSampleBuffer? {
        let audioBufferList = pcmBuffer.mutableAudioBufferList
        let asbd = pcmBuffer.format.streamDescription

        var sampleBuffer: CMSampleBuffer? = nil
        var format: CMFormatDescription? = nil
        
        var status = CMAudioFormatDescriptionCreate(allocator: kCFAllocatorDefault,
                                                         asbd: asbd,
                                                   layoutSize: 0,
                                                       layout: nil,
                                                       magicCookieSize: 0,
                                                       magicCookie: nil,
                                                       extensions: nil,
                                                       formatDescriptionOut: &format);
        if (status != noErr) { return nil; }
        
        var timing: CMSampleTimingInfo = CMSampleTimingInfo(duration: CMTime(value: 1, timescale: Int32(asbd.pointee.mSampleRate)),
                                                            presentationTimeStamp: CMClockGetTime(CMClockGetHostTimeClock()),
                                                            decodeTimeStamp: CMTime.invalid)
        status = CMSampleBufferCreate(allocator: kCFAllocatorDefault,
                                      dataBuffer: nil,
                                      dataReady: false,
                                      makeDataReadyCallback: nil,
                                      refcon: nil,
                                      formatDescription: format,
                                      sampleCount: CMItemCount(pcmBuffer.frameLength),
                                      sampleTimingEntryCount: 1,
                                      sampleTimingArray: &timing,
                                      sampleSizeEntryCount: 0,
                                      sampleSizeArray: nil,
                                      sampleBufferOut: &sampleBuffer);
        if (status != noErr) { NSLog("CMSampleBufferCreate returned error: \(status)"); return nil }
        
        status = CMSampleBufferSetDataBufferFromAudioBufferList(sampleBuffer!,
                                                                blockBufferAllocator: kCFAllocatorDefault,
                                                                blockBufferMemoryAllocator: kCFAllocatorDefault,
                                                                flags: 0,
                                                                bufferList: audioBufferList);
        if (status != noErr) { NSLog("CMSampleBufferSetDataBufferFromAudioBufferList returned error: \(status)"); return nil; }
        
        return sampleBuffer
    }
}

Apple docs for CMSampleBufferCreate state:

Example of usage for uncompressed (non-)interleaved audio:
...
sampleTimingArray: one entry = {duration = 1/48000, presentationTimeStamp = 0/48000, decodeTimeStamp = invalid }
...

So I think the definition of the timing parameter should be:

var timing: CMSampleTimingInfo = CMSampleTimingInfo(
    duration: CMTime(value: 1, timescale: Int32(asbd.pointee.mSampleRate)),
-   presentationTimeStamp: CMClockGetTime(CMClockGetHostTimeClock()),
+   presentationTimeStamp: CMTime(value: 0, timescale: Int32(asbd.pointee.mSampleRate)),
    decodeTimeStamp: CMTime.invalid
)

That doesn't seem correct as each buffer will then have 0 as their timestamp causing issues when writing to disk.