Как Вы открываете веб-страницы в Java?

создайте папку 'upload/files' внутри хранилища / app / public и /upload/files/audio

запустите команду: php artisan storage: ссылка

эта команда будет свяжите вашу папку хранения с общей папкой

обновите ваш код:

namespace App\Http\Controllers\Api;

use Illuminate\Http\Request;
use App\Http\Controllers\Controller;
use Illuminate\Support\Facades\Storage;
use App\File;
use Illuminate\Support\Facades\DB;
use Illuminate\Support\Facades\Response;
use Illuminate\Support\Facades\URL;
use Illuminate\Support\Facades\Auth;
use Carbon\Carbon;

public function store(Request $request)
    {
        // validation
        $this->validate($request, [
            'title' => 'required',
            'description' => 'nullable',
            'file' => 'required|file|mimes:jpeg,jpg,png,gif|max:2048',
            'audio' =>'nullable|file|mimes:audio/mpeg,mpga,mp3,wav,aac'
        ]); 

         // code for upload 'file'

          if($request->hasFile('file')){
            $uniqueid=uniqid();
            $original_name=$request->file('file')->getClientOriginalName();
            $size=$request->file('file')->getSize();
            $extension=$request->file('file')->getClientOriginalExtension();
            $name=Carbon::now()->format('Ymd').'_'.$uniqueid.'.'.$extension;
            $imagepath=url('/storage/uploads/files/'.$name);
            $path=$request->file('file')->storeAs('public/uploads/files/',$name);  
           }


        // code for upload 'audio'
        // handle multiple files 
       if(is_array($request->file('audio')))
        {
         $audios=array();
         foreach($request->file('audio') as $file) {
            $uniqueid=uniqid();
            $original_name=$file->getClientOriginalName();
            $size=$file->getSize();
            $extension=$file->getClientOriginalExtension();
            $filename=Carbon::now()->format('Ymd').'_'.$uniqueid.'.'.$extension;
            $audiopath=url('/storage/upload/files/audio/'.$filename);
            $path=$file->storeAs('/upload/files/audio',$filename);
            array_push($audios,$audiopath);
         }
         $all_audios=implode(",",$audios);
     }else{ 
         // handle single file 
         if($request->hasFile('audio')){
         $uniqueid=uniqid();
         $original_name=$request->file('audio')->getClientOriginalName();
         $size=$request->file('audio')->getSize();
         $extension=$request->file('audio')->getClientOriginalExtension();
         $filename=Carbon::now()->format('Ymd').'_'.$uniqueid.'.'.$extension;
         $audiopath=url('/storage/upload/files/audio/'.$filename);
         $path=$file->storeAs('public/upload/files/audio/',$filename);
         $all_audios=$audiopath;
        }
    }

}

в вашем запросе почтальона:

add key : "file"  for image file , 
          "audio" for audio file