Dear Customer,
Thank you for contacting SEGA Customer Support.
We are sorry to read that you are encountering an issue with Sonic Mania.
Firstly please reverify the game cache of Sonic Mania in Steam:
1. Load Steam
2. From the Library section, right-click on the game and select Properties from the menu.
3. Select the Local files tab and click the ‘VERIFY INTEGRITY OF GAMECACHE...' button.
4. Steam will verify the game's files - this process may take several minutes.
5. Once the process is completed, the Check Window will automatically exit.
6. We recommend verifying your cache twice to make sure all files are fully downloaded
Also please make sure that you are having the latest drivers installed for your graphics and sound chipset/ cards.
For Nvidia cards please see here:
http://www.geforce.co.uk/drivers
For AMD Radeon/ ATi cards:
http://support.amd.com/us/gpudownload/Pages/index.aspx
For integrated Intel chipsets:
http://downloadcenter.intel.com/default.aspx?lang=eng
Important: Please double check that all your Windows Updates are fully installed and completely up to date.
https://support.microsoft.com/en-us/help/12373/windows-update-faq
Please let us know how you get on.
Best Regards,
Vincent
SEGA Customer Support