I managed to fix my "no sound with my Lubuntu install inside VirtualBox" issue, which I will explain below. If your settings differ from mine, my solution might still work for you so give it a try anyway. I am using VirtualBox Version 6.0.8 r130520 (yes, I use a rather old version for stability), Lubuntu 19.1 Eoan, and Guest Additions 6.0.8. If you ever wonder how to check your Ubuntu/Lubuntu version, Guest Addition versions, etc., here's how:
$ uname -a
Linux my-pc 5.3.0-23-generic #25-Ubuntu SMP Tue Nov 12 09:22:33 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
$ lsb_release -a
Description: Ubuntu 19.10
Codename: eoan
$ ls /opt
net.downloadhelper.coapp VBoxGuestAdditions-6.0.8
Your first step should be to make sure you installed the Guest Additions. For me, that didn't resolve my issue. The additional step I take is to change the audio card of my virtual machine from ICH AC97 to Intel HD. For me, that resolves the sound issue.
Another additional wrinkle here is that I did have to reboot the machine with both audio output and input (if you are recording) enabled in the VirtualBox settings. I assume the cause is that if Lubuntu detects the audio output/input disabled, it doesn't load some audio drivers.
Good luck with your sound issue!