Hey udayakantha,
We understand you're able to connect your Marshall Stanmore II speaker to your iPhone, but there's no audio output. We're happy to help!
When you installed the app, and connected the speaker did you notice a pop-up asking for permission for the app to use Bluetooth? Let's check that by tapping Settings > Privacy > Bluetooth. Do you see the Marshall app listed here? If so, make sure it has permission.
Also be sure the speaker is chosen as the audio source by opening the Control Center (swiping down from upper-right corner), then taking a look at the audio card that is also in the upper-right corner. If it doesn't show the speaker, press and hold it to open the card and choose the speaker. More details on that can be found under "AirPlay audio from your iPhone, iPad, or iPod touch" here: How to AirPlay audio
It's also a great idea to be sure you're fully updated with your software and apps. The current version of iOS is 13.6.1. If that's not what you show in Settings > General > About, you'll want to back up your iPhone, then update the software and apps. Check out those steps here:
Update your iPhone, iPad, or iPod touch
then
Update apps or use automatic downloads
We hope this helps. Cheers!