The OnePlus 6 comes with a single, bottom-firing speaker, though thanks to a new mod, you can utilize the phone's earpiece as a speaker as well. If you're looking to boost audio on the OnePlus 6, XDA Senior Member, 'acervenky', has created a mod for the phone which will turn an earpiece in a secondary speaker, basically, and that will give you a front-facing speaker on the phone as well.
Do keep in mind that in order to load this mod on the OnePlus 6, you'll need Magisk in order to enable the module itself, and the source notes that it will work partially on a stock version of OxygenOS. If you'd like to get the stereo audio working along with this mod, you will need to load the NoLimits ROM or Viper Audio on the OnePlus 6. So, basically, if you have no experience tweaking smartphones, and you don't feel safe enough to do it on your own, it would be best if you don't do it at all, as things can go south and you may end up messing up the OnePlus 6's software and as a result brick the device. This is not exactly a simple install, and things can go wrong.
That being said, if you'd like to risk it anyway, visit the source link down below for all the info you'll need, and some files that you'll need to install in order to get the mode working. In addition to all that, a user who developed this mod, 'acervenky', also shared a tutorial video on his YouTube page, though do keep in mind that he already has Magisk installed on the OnePlus 6 and all the prerequisites in order to make this work, it's not a full tutorial video how to get everything set up if your phone is running stock and you haven't installed any modding tools on it. Having said that, the tutorial video has been embedded down below for your convenience, just in case you opt to try out this mod.