Responsa for Rosh Hashanah 54:1

ניקב וסתמו אם מעכב את התקיעה פסול ואם לאו כשר התוקע לתוך הבור או לתוך הדות או לתוך הפיטס אם קול שופר שמע יצא ואם קול הברה שמע לא יצא

IF A HOLE IN A SHOFAR HAS BEEN STOPPED UP, IF IT INTERFERES WITH THE BLOWING IT IS NOT VALID, BUT OTHERWISE IT IS VALID.<span class="x" onmousemove="('comment',' The Talmud Yerushalmi reads here, 'If it (the hole) interfered with the blowing before it was closed, the shofar is not valid after it was closed'. cehb Our version, however, rather implies that if the stoppage restores the shofar to its original condition, it may be used. V. Tosaf. s.v. ,us');">¹</span> IF ONE BLOWS INTO A PIT OR A CISTERN<span class="x" onmousemove="('comment',' Heb. , a pit faced with cement.');">²</span>

Teshuvot Maharam