Basically Ravinder Kumar is right. Because you seem new to flutter, I've made an easy to understand example based on the sample code generated when a new project is created. So my advise would be to generate a new project and paste my code, in order to see how you need to modify your app. I would recommend using a StatefulWidget, as I did.

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  void _showDialog() {
    // flutter defined function
    showDialog(
      context: context,
      builder: (BuildContext context) {
        return AlertDialog(
          title: new Text("Alert Dialog title"),
          content: new Text("Alert Dialog body"),
          actions: <Widget>[
            new FlatButton(
              child: new Text("Close"),
              onPressed: () {
                Navigator.of(context).pop();
              },
            ),
          ],
        );
      },
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'Push the FAB to display a dialog.',
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _showDialog,
        tooltip: 'Show Dialog',
        child: Icon(Icons.add),
      ),
    );
  }
}

EDIT: I took your code and inserted a dialog in the fab...

import 'package:audioplayers/audio_cache.dart';
import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(debugShowCheckedModeBanner: false, home: HomePage()));
}

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      bottomNavigationBar: BottomAppBar(
        child: Text(
          "test bottom",
          style: TextStyle(
            color: Color(0xffFFFFFF),
            fontSize: 10,
          ),
          textAlign: TextAlign.center,
        ),
        color: Colors.blueGrey[600],
      ),
      backgroundColor: Colors.lightBlueAccent[50],
      appBar: AppBar(
        title: Text('test'),
        backgroundColor: Colors.blueGrey[600],
      ),
      body: SoundTest(),
      floatingActionButton: FloatingActionButton(
        backgroundColor: Colors.blue,
        child: Icon(Icons.priority_high),
        onPressed: () {
          showDialog(
            context: context,
            builder: (BuildContext context) {
              // return object of type Dialog
              return AlertDialog(
                title: new Text("Alert Dialog title"),
                content: new Text("Alert Dialog body"),
                actions: <Widget>[
                  // usually buttons at the bottom of the dialog
                  new FlatButton(
                    child: new Text("Close"),
                    onPressed: () {
                      Navigator.of(context).pop();
                    },
                  ),
                ],
              );
            },
          );
        },
      ),
    );
  }
}

class SoundTest extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Column(
      crossAxisAlignment: CrossAxisAlignment.stretch,
      children: <Widget>[
        Expanded(
          child: FlatButton(
            child: Image.asset('images/softsound.png'),
            onPressed: () {
              final player = AudioCache();
              player.play('clicksoft.wav');
              //I do not know what this is supposed to do...
              //If you want to show the dialog here, simply copy/paste from above...
              /*showDialog(context: context,
          );*/
            },
          ),
        ),
        Expanded(
          child: FlatButton(
              child: Image.asset('images/loudsound.png'),
              onPressed: () {
                final player = AudioCache();
                player.play('clickloud.wav');
              }),
        )
      ],
    );
  }
}